Infer type var tuple contents in more situations

[A5rocks]

Supersedes #17512 as I figured out how to handle this case in a more general manner.

This PR fixes #16522. It adds a mechanism for constraints to be discarded if they are less likely to be correct: for instance, if you pass f(x: int, y: str = "42") to Callable[[Unpack[Ts]], None] it's not clear whether y should be included in an unpacked TypeVarTuple. This PR makes it such that function_using_typevartuple(f, 42) discards constraints based on f.

[A5rocks]

The only bad change in mypy primer is:

+ homeassistant/components/homekit/accessories.py:656: error: Argument 2 to "async_dispatcher_send" has incompatible type "SignalType[tuple[str, ...]]"; expected "SignalType[tuple[str]]"  [arg-type]

... and I'm not quite sure if there's a good way to solve this. We could check whether constraints interfere with each other and only if they do prefer the higher quality ones? That sounds slow, though. (EDIT: I did that, but again I'm not confident it's right)

[A5rocks]

@ilevkivskyi this is my best guess at a mechanism that would handle TypeVarTuple inference when there's also kwargs that need to be discarded. Does it seem reasonable?

[github-actions]

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

sockeye (https://github.com/awslabs/sockeye)
+ sockeye/layers.py:530: error: Unused "type: ignore" comment  [unused-ignore]
+ sockeye/layers.py:873: error: Unused "type: ignore" comment  [unused-ignore]

core (https://github.com/home-assistant/core)
+ homeassistant/util/signal_type.pyi:57: error: Unused "type: ignore" comment  [unused-ignore]
+ homeassistant/util/signal_type.pyi:57: error: Argument 2 to "async_dispatcher_send" has incompatible type "SignalType[int]"; expected "SignalType[str]"  [arg-type]
+ homeassistant/util/signal_type.pyi:57: note: Error code "arg-type" not covered by "type: ignore" comment