dict: on retrieval, key allowed to be null

[refack]

> uv run --with pyright pyright  .\dict-none-key.py 
e:\1work\CFF\dict-none-key.py
  e:\1work\CFF\dict-none-key.py:4:12 - error: Argument of type "int | None" cannot be assigned to parameter "key" of type "int" in function "get"
    Type "int | None" is not assignable to type "int"
      "None" is not assignable to "int" (reportArgumentType)

But

> python -c "a = {'A': 1};b = {1: 'א'};k1 = a.get('A');k2 = b.get(k1);print('no runtime error')"                                                                                                                                   
no runtime error      

[refack]

It just came to me that this might have been a design decision as:

a = {'A': 1}
b: dict[int | None, str] = {1: 'א'}
k1 = a.get('A')
k2 = b.get(k1)

gives:

> uv run --with pyright pyright  .\dict-none-key.py                                             
0 errors, 0 warnings, 0 informations

I searched manually, and with copilot and found no direct mention...

Is this PR acceptable in principal, which in that case I'll cross all the I's and dot all the T's

[JelleZijlstra]

Type aliases defined like this are technically questionable, so let's just write _KT | None where appropriate.

[JelleZijlstra]

It's sort of questionable to me to treat None specially here. I think we've had someone before trying this (specifically for dict.get) and we rejected it, but can't immediately find it back. dict.get has seen a lot of efforts to change things: https://github.com/python/typeshed/pulls?q=is%3Apr+is%3Aclosed+dict.get+