Add typing for context managers and methods returning Self in non-generated python code

[echoix]

This is a smaller, conservative PR that doesn't step on the toes of #2668.
It adds a return type annotation for typing.Self, introduced in Python 3.11, or coming from typing_extensions otherwise. https://typing-extensions.readthedocs.io/en/latest/#typing_extensions.Self
https://docs.python.org/3/library/typing.html#typing.Self

It is for non-generated files.

I did an attempt to generate it for all the context managers via the etg scripts, but while it worked, it imported multiple times the Self throughout the files (the runtime ones, not only pyi), as I only found how to use c.addPyCode(), which isn't exactly appropriate to define an import that should be deduplicated and placed at top. Maybe SIP has a more specialized directive for that that would allow deduplicating these imports when combining different files together. So I simply did not include it yet.

Something like this, where the imports are not deduplicated, and can appear multiple times:

    # context manager methods
    c.addPyCode("""\
import sys
if sys.version_info >= (3, 11):
    from typing import Self
else:
    from typing_extensions import Self
    """)
    c.addPyMethod('__enter__', '(self) -> Self', 'return self')

[swt2c]

Thanks a lot, rebased due to merge conflict with your other PR.

[lojack5]

FYI: <3.10 still needs typing_extensions due to the use of typing.ParamSpec.

[echoix]

This PR has it bumped higher since Self was introduced in 3.11, so we're good

[lojack5]

Oh right, my math brain was backwards.