[mypy] Fix has no attribute errors

These attributes (expression, operator, boundary) are specific to some kind of constraints.
We shouldn't expect them to be present in every constraints.
Let's keep the type ignore in diff.py for the moment.

yams/diff.py

@@ -142,7 +142,7 @@ def format_constraint(cstr: IConstraint) -> str:
     cclass = cstr.__class__.__name__
 
     if "RQL" in cclass:
-        out = ["%s(%s" % (cclass, quoted(cstr.expression))]
+        out = ["%s(%s" % (cclass, quoted(cstr.expression))]  # type:ignore[attr-defined]
         mainvars = getattr(cstr, "mainvars", None)
 
         if mainvars:
@@ -150,7 +150,11 @@ def format_constraint(cstr: IConstraint) -> str:
 
         out.append(")")
     elif "Boundary" in cclass:
-        return "%s(%s %s)" % (cclass, quoted(cstr.operator), str(cstr.boundary))
+        return "%s(%s %s)" % (
+            cclass,
+            quoted(cstr.operator),
+            str(cstr.boundary),
+        )  # type:ignore[attr-defined]
 
     else:
         print("formatter: unhandled constraint type", cstr.__class__)