Loosen Mapping.update() overloads a little

[AlexWaygood]

We received a report in ty that said that the changes made in #14099 are now too restrictive. Mypy and ty will now reject a call like this, which feel like it should be valid because object is obviously a supertype of str:

def f(x: dict[object, object]):
    x.update(foo="bar")

the call is rejected because Mapping is invariant in its key type, meaning that mypy and ty cannot select this overload: dict[object, object] is not assignable to Mapping[str, _VT]:

typeshed/stdlib/typing.pyi

Lines 809 to 810 in ab0968b

	@overload
	def update(self: Mapping[str, _VT], **kwargs: _VT) -> None: ...

this PR uses a contravariant protocol for the self annotations in the MutableMapping.update() overloads, so that this overload can now be selected if the mapping has a key type that is str or any supertype of str. I think this should be sound?

Fixes python/mypy#10970, fixes astral-sh/ty#1048

[github-actions]

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

aioredis (https://github.com/aio-libs/aioredis)
+ aioredis/client.py:4116: error: Unused "type: ignore" comment  [unused-ignore]
+ aioredis/client.py:4152: error: Unused "type: ignore" comment  [unused-ignore]

discord.py (https://github.com/Rapptz/discord.py)
- discord/ext/commands/hybrid.py:508: error: Overlap between argument names and ** TypedDict items: "name", "description"  [misc]
+ discord/ext/commands/hybrid.py:508: error: Overlap between argument names and ** TypedDict items: "description", "name"  [misc]
- discord/ext/commands/hybrid.py:629: error: Overlap between argument names and ** TypedDict items: "name", "description"  [misc]
+ discord/ext/commands/hybrid.py:629: error: Overlap between argument names and ** TypedDict items: "description", "name"  [misc]

[AlexWaygood]

Nice -- the aioredis hits look like a false positive going away! https://github.com/aio-libs-abandoned/aioredis-py/blob/19be499015a8cf32580e937cbfd711fd48489eca/aioredis/client.py#L4105-L4116