update undo_manager property, type:
  self._undo_manager as SelectionManager
and fix:
  rql/stmts.py:424: error: Cannot determine type of "_undo_manager" [has-type]

rewrite the undo_manager function and make sure it always returns a SelectionManager

- remove even more useless comments
- remove even more useless casts

if we want to use it as 2nd arg to the property function:
- set_limit method should explicitly return None
- remove useless comments
- remove now useless type:ignore comments

This fix mypy error:
Argument 2 to "property"
 has incompatible type "Callable[[rql.stmts.Union, Any], Optional[rql.stmts.Union]]"
 expected "Optional[Callable[[Any, Any], None]]"

- annotate schema as ISchema
- remove useless comments
- remove useless cast
- remove now useless type:ignore comments

- `visit_insert` method should only accept rql.stmts.Insert type nodes
- `visit_delete` should only accept rql.stmts.Delete type nodes

- improve type aliases, Select is not a statement
- pick better names for type aliases

The `_visit` method receive a node as first parameter.
Annotate `node`:
- either as a union of all 18 possible node types
- or just type node as BaseNode (picked solution)

There was an unused protocol definition sleeping within the rql/interfaces.py module.
This protocol (ISchema) perfectly represent what a schema should be.
Let's use it to annotate the schema attribute within the RQLHelper class

Within rql/analyze.py:
----------------------
  add (line 403):
      if self.uid_func:
  and fix:
      error: "None" not callable  [misc]

  add (line 548):
      assert self.uid_func_mapping is not None
  and fix:
      Unsupported right operand type for in ("Optional[Dict[Any, Any]]")

  change (line 615):
      etypes = frozenset(t for t in self._nonfinal_domain if t not in etypes)
  to:
      etypes = set(t for t in self._nonfinal_domain if t not in etypes)
  and fix:
      Incompatible types in assignment (expression has type "FrozenSet[str]", variable has type "Set[Any]")


Within rql/interfaces.py:
-------------------------

Update ISchema protocol definition:
    add:
        def __contains__():
    and fix:
         error: Unsupported right operand type for in ("ISchema")  [operator]

    add:
        def rschema(self, rtype: Any):
    and fix:
        "ISchema" has no attribute "rschema"  [attr-defined]

    add:
        def eschema(self, etype: Any):
    and fix:
        "ISchema" has no attribute "eschema"  [attr-defined]


See RQL's issue: #15

To easily build the wheel using `build_wheel.sh` shell script

In order to upload a RQL build using the (faster) gecode library.

- Retrieve a copy of Yapps repository
- Fix the tiny bugs related to the use of the print function
- From now on, install Yapps from Logilab's forge

fixes #12

There is some cases where the `get_description` method will return None.
The TranslationFunction assume the first parameter will be a string, not None.

Alternative solutions:
- update the condition: vtype is not None and != "Any" (picked solution)
- assert returned value is not None whenever necessary
- update TranslationFunction signature to take first parameter as Optional[str].
- update all the `get_description` methods to always return a str.

- define necessary attributes within a protocol
- this protocol should have:
   - operator attribute
   - iget_nodes method
   - get_type method
- use the newly defined protocol to type `self` within OperatorExpressionMixin class
- remove useless type-ignore comments

Inside rqltypes module
- define a protocol to describe an object that:
  - has an attribute named `parent`
  - parent attribute should have an attribute named `relation`
  - a method named `get_type`

Inside HSMixin:
- remove now useless type:ignore comments
- type `self` as a `HasHand` type

Inside nodes.Constant `get_type` method could return a str or None:
- make sure `get_type` always returns a str
- change the default value of `_uidtype` from None to empty string
- type `_uidtype` as a str

- Update Gecode version to the last one
- To build rql wheels, starts with Python 3.7 version
- Use pip flag --use-feature=in-tree-build, and deal with the following warning:

    DEPRECATION: A future pip version will change local packages to be built
    in-place without first copying to a temporary directory. We recommend you use
    --use-feature=in-tree-build to test your packages with this new behavior
    before it becomes the default.
       pip 21.3 will remove support for this functionality. You can find
    discussion regarding this at https://github.com/pypa/pip/issues/7555.

Inside stmts module,
somewhere we read, `self.groupby` attribute being added if not exists and used as a list:
```
if not self.groupby:
    self.groupby = []
```

Let's just define:
- self.groupby self.orderby as instance attribute
- self.groupby self.orderby as List of Any
- remove the if not self.grouby check

Do the same with the `self.orderby` attribute.

Inside Select class:
- update `remove` method signature. And fix the error:
  rql/stmts.py:940: error:
  Argument 1 to "remove_sort_term" of "Select" has incompatible type "BaseNode";
  expected "SortTerm"  [arg-type]

- update undefine_variable method:
  remove useless check of var.variable attribute

Inside undo.NodeOperation class:
- simplify type hint for `node` parameter inside `NodeOperation.__init__` method

- Type hint all `parse` functions inside rql/__init__.py
- Create a type alias to represent what's returned by `parse` functions above
- Update RQLSTChecker.check method signature:
  * node parameter should be a BaseNode	or any children	of BaseNode class
- Type hint `copy_uid_node` function
- Remove useless variable: pyparse = parse
- Format without any compromise

Update rql.base.Node class:
- define self.parent as a TypeVar
- define self.children as a List of TypeVar
- Those 2 TypeVar are bound, i.e: they can be BaseNode or any children of BaseNode

Update rql.stmts.Union class:
- add a new attribute, i.e:  `children: List["rql.stmts.Select"]`

Update rql.RQLHelper simplify & _simplify methods:
- remove call to cast method, no more need to cast from BaseNode to Select

Also:
- Try to harmonize type signature for get_description methods
- Remove now obsolete comments related to type hints
- type vconsts as a List of Constant