add deduplication of types

[walter9388]

Relating to #982

Took me a while to get round to this, but here we are....

I think there are three different levels of approaching this issue:

1. Remove extra None types only:

   -def f(x: Optional[Union[int, None]]): pass
   +def f(x: int | None): pass
   -def g(x: Union[Optional[int], None]): pass
   +def g(x: int | None): pass

2. Remove any duplicated scalar types at the same depth by name in Union blocks:

   -def f(x: Union[Union[Union[Union[a, b], c], d], a]): pass
   +def f(x: a | b | c | d): pass
   -def g(x: Union[a.b | a.c, a.b, list[str], str]): pass
   +def g(x: a.b | a.c | list[str] | str): pass

3. General deduplication at any depth on any block:

   -def f(x: Union[list[Union[int, str]], list[Union[str, int]]]): pass
   +def f(x: list[int | str]): pass

I settled on level 2 as this was still possible with a single pass and seemed more useful than just focusing on None types.
I couldn't see a way of approaching the general problem (level 3) without recursively making a tree structure and then assessing the leaf nodes. However, I am not deeply familiar with the standard python libraries for parsing ASTs etc., so if there are simple built in methods for problems like this I would be interested to know!

I used the existing scan in _fix_union to determine the delimitators at depth==1 between the types. This seemed to work well, but I definitely ran into some interesting edge cases when it came to handling comments, whitespace and multilines.

I have managed to get this working for a variety of test cases, and I would be interested to hear your feedback.

Btw I enjoy your YouTube content! I have learnt a lot of niche things I would have struggled to pick up otherwise. So thank you for that!

[asottile]

just a quick first pass -- will look more closely later

[walter9388]

Just highlighting a few things to be aware of.

[walter9388]

Modified the general find_closing_bracket function to also check for whether the optional block already contains None in the same pass.

[walter9388]

The reason for changing the single token containing | None to explicit whitespace, | and None is for the deduplication and whitespace removal functions used in _fix_union. This also applies to the multiline version a few lines below.

[walter9388]

Not convinced this is the best approach to remove whitespace, but not sure about what to do in situations where lines are completely deleted other than comments. I have written a niche test for this situation in test id='duplicated types in multi-line nested unions or optionals'.