Use TypeAlias in builtins and complete a TODO item

[sobolevn]

From #9158 I understood that explicit type aliases are not a goal. But, this one is special. It has a TODO item and some history to it.

I am also curious about whether mypy now supports it.

[sobolevn]

stdlib/builtins.pyi:195: error: Unsupported left operand type for | ("object") [operator]

Not today!

[github-actions]

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

Expression (https://github.com/cognitedata/Expression)
+ expression/collections/block.py:317: error: Incompatible return value type (got "int", expected "Union[_TSourceSum, Literal[0]]")  [return-value]
+ expression/collections/block.py:1036: error: Incompatible return value type (got "int", expected "Union[_TSourceSum, Literal[0]]")  [return-value]
+ expression/collections/block.py:1043: error: Incompatible return value type (got "int", expected "Union[_TSourceSum, Literal[0]]")  [return-value]

ignite (https://github.com/pytorch/ignite)
+ ignite/handlers/time_profilers.py:607: error: Incompatible types in assignment (expression has type "float", variable has type "int")  [assignment]

[AlexWaygood]

I am also curious about whether mypy now supports it.

I haven't checked yet, but I wonder if python/mypy#14008 might have fixed this bug (not yet released, will be released as part of mypy 1.0).