Support unions in functools.partial

[hauntsaninja]

Co-authored-by: cdce8p

[github-actions]

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

websockets (https://github.com/aaugustin/websockets)
+ src/websockets/legacy/server.py:1048: error: "type[WebSocketServerProtocol]" not callable  [misc]
+ src/websockets/legacy/client.py:496: error: "type[WebSocketClientProtocol]" not callable  [misc]

[cdce8p]

Looks good! I like how it is now part of extract_callable_type and thus potentially other code can benefit from it too, not just the functools plugin 👍🏻

[cdce8p]

@hauntsaninja Looking at the primer result, that seems to suggest another issue.

class A:
    def __init__(self, val: int) -> None: ...

cls: type[A]
reveal_type(functools.partial(cls, 2))  # error: "type[A]" not callable  [misc]