Improve type narrowing for walrus operator in conditional statements

[hauntsaninja]

Authored by @kprzybyla in #9288. Helps with issue mentioned in #9229.

I fixed some nits and tests, but Github didn't like it when I tried to push to the original branch, so opening a new PR.

[github-actions]

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

aioredis (https://github.com/aio-libs/aioredis.git)
- aioredis/client.py:3756: error: Argument 1 to "append" of "list" has incompatible type "Union[bytes, memoryview, str, int, float, None]"; expected "Union[Union[bytes, memoryview], Union[str, int, float]]"
- aioredis/client.py:3771: error: List item 1 has incompatible type "Union[bytes, memoryview, str, int, float, None]"; expected "Union[bytes, str, float]"
- aioredis/client.py:3785: error: List item 1 has incompatible type "Union[bytes, memoryview, str, int, float, None]"; expected "Union[bytes, str, float]"
- aioredis/client.py:3788: error: List item 1 has incompatible type "Union[bytes, memoryview, str, int, float, None]"; expected "Union[bytes, str, float]"

rich (https://github.com/willmcgugan/rich.git)
+ rich/text.py:559: error: Right operand of "or" is never evaluated
+ rich/text.py:563: error: Right operand of "or" is never evaluated
+ rich/text.py:1068: error: Right operand of "or" is never evaluated
+ rich/text.py:1070: error: Right operand of "or" is never evaluated

[hauntsaninja]

Going ahead and merging, since all feedback in the original PR has been addressed. The mypy_primer changes all look correct to me (and seem to be the result of the new fallback case for type maps).

[hauntsaninja]

Thank you for this, kprzybyla!