Some typing_extensions rewrites are perhaps overly aggressive

[AlexWaygood]

Hi, thanks so much for pyupgrade -- I think it's a fantastic tool.

When run with e.g. the --py39-plus flag, pyupgrade will currently make the following changes, which are perhaps overly aggressive:

-from typing_extensions import Protocol
-from typing_extensions import SupportsIndex
-from typing_extensions import NewType
+from typing import Protocol
+from typing import SupportsIndex
+from typing import NewType

In the latest version of typing_extensions, several performance improvements (and several bugfixes) from Python 3.12 have been backported to typing_extensions.Protocol. The upshot of this is that isinstance() checks against runtime-checkable protocols may be 20x faster or more if you're using typing_extensions.Protocol. The same rationale applies for why a user might want to use typing_extensions.SupportsIndex rather than typing.SupportsIndex.

The latest version of typing_extensions also backports the implementation of typing.NewType from Python 3.10 to older Python versions. In Python 3.10, NewType is implemented as a class, which makes newtypes pickleable; on earlier versions of Python, newtypes were not pickleable. (This backport was done in response to a request from a user who was struggling to use multiprocessing with typing.NewType on older Python versions due to the fact that newtypes could not be pickled.)

[asottile]

if you want to update the symbols they're generated in https://github.com/asottile/reorder-python-imports -- there's already several carveouts for typing-extensions there as well so it's easy to add more