These types are not supposed to be accessed by client code.

TypeVars should be referenced at least twice otherwise they have no effect.

Add a job that will:
- clone the last Cubicweb version
- update rql requirement within Cubicweb's setup.py
- finally, run tox -e py3-server

See #8

Searched across all projects on the forge.

This allows to support pyright which doesn't support sub-typing of conditional types.
See https://github.com/microsoft/pyright/issues/2047.

Not a breaking change since no version has been released.

BREAKING CHANGE: this shouldn't affect projects since updating rql will automacally install the new dependency.

Without this, we have an error if we have aggregate functions indise other
function.

related: cubicweb#242

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)