Support Python 3.10a5 or 3.9x with support explicitly enabled.

• Enable Python's postponed annotations for Blender's RNA classes types registered on startup.

• Using postponed annotations has implications for how they are defined, since they must evaluate in the modules name-space instead of the classes name-space. See changes to annotations in release/scripts.

• Use from __future__ import annotations at the top of the module to ensure the script will run with Python 3.10.

• Old logic is kept since it could be used if PEP-649 is supported.

Resolves T83626

This is an initial patch to get Python 3.10 working with annotations.

Other commits for Python 3.10 support which have been committed to master.

• rBeafcbbb4f741: PyAPI: use real module for bpy.types
• rBeecb90d8d2ec: PyAPI: bpy.utils.execfile temporarily overrides the __main__ module
• rB37e6a1995ac7: PyAPI: use a new type for storing the deferred result of bpy.props