Drop support for mypy_extensions.TypedDict

[davidfstr]

Why:

• This kind of TypedDict was introduced in the era of Python 3.7, which is no longer supported by trycast.
  • It is reasonable (in @davidfstr 's opinion) to encourage other users to migrate to a version of TypedDict from the standard library (Python 3.9+) or from typing_extensions (Python 3.8).
• Extra code is required to support this very old kind of TypedDict.

Timing:

• Suggest actually dropping mypy_extensions.TypedDict support at the same time as dropping support for Python 3.8, because then the trycast(strict=...) parameter (which involves both Python 3.8's TypedDict and mypy_extensions.TypedDict, and nothing else) can be cleanly removed simplified at the same time

[davidfstr]

mypy_extensions.TypedDict was deprecated in Aug 2023, according to python/mypy_extensions#46