Mark typing.TYPE_CHECKING as Final

[brianschubert]

Per PEP 563, typing.TYPE_CHECKING is intended to be a constant. It seems sensible to annotate it as such.

(Apparently some projects do modify it (e.g. sphinx autodoc temporarily sets it to True to help with import analysis), but that sort of use case seems dubious/exceptional enough to warrant a # type: ignore.)

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

sphinx (https://github.com/sphinx-doc/sphinx)
+ sphinx/ext/autodoc/importer.py: note: In function "import_object":
+ sphinx/ext/autodoc/importer.py:182:25: error: Cannot assign to final name "TYPE_CHECKING"  [misc]
+ sphinx/ext/autodoc/importer.py:188:25: error: Cannot assign to final name "TYPE_CHECKING"  [misc]

[TeamSpen210]

It should probably be a literal too?

[srittau]

I don't think we should make this a literal. This will be special cased by type checkers anyway, but it's unclear whether it should be True or False in the stubs, i.e. whether the stubs should represent runtime or type checking time.