Tool for generating schema type annotations

[aljazerzen]

Our schema classes have generated getter methods for each of the fields:

class Pointer:
    source = so.SchemaField(
        so.InheritingObject,
        default=None, compcoef=None,
        inheritable=False
    )
 

    # this is auto generated by a meta class in objects.py
    def get_source(schema: s_schema.Schema) -> objects.InheritingObject: ...

This PR adds a tool that generates .pyi files in edb/schema/, which contain definitions for all of these generated getters.

---

We do have mypy extension that already picks-up these getters, even without .pyi definitions.

Regardless, this .pyi approach is useful because language servers (PyLance) cannot use mypy extension. Now, "go to definition", "show usages" and other code actions work.

There is a few drawbacks:

• PyLance will not look at .pyi file when editing associated .py file. They suggest merging the two files.
• In this example:

  subjexpr = constr.get_subjectexpr(ctx.env.schema)

  ... PyLance will understand that get_subjectexpr returns Expression, but it will infer type of subjexpr to be Any. What?

Because of these drawbacks, #7166 is still useful.

[aljazerzen]

The verdict here is:

• we want this, but working better. This would best be done with these .pyi files being actual
  .py files in edb/schema/generated/ and the base classes adding the generated type-annotation classes as a mixin.

  # edb/schema/pointers.py
  
  class Pointer(..., PointerMixin):
      source = so.SchemaField(
        so.InheritingObject,
        default=None, compcoef=None,
        inheritable=False
      )
  
  # edb/schema/generated/pointers.py
  class PointerMixin:
      def get_source(schema: Schema) -> so.InheritingObject: ...

• alternatively, we could stop creating these functions at run-time and just generate them in the mixin classes:

  # edb/schema/pointers.py
  
  class Pointer(..., PointerMixin):
      source = so.SchemaField(
        so.InheritingObject,
        default=None, compcoef=None,
        inheritable=False
      )
  
  # edb/schema/generated/pointers.py
  class PointerMixin:
      def get_source(self, schema: Schema) -> so.InheritingObject:
          self.get_field_value('source', schema)