gh-127750: Fix singledispatchmethod caching (v2)

[eendebakpt]

Version based on idea from @dg-pb in #127839. This version

• Fixes the issue of collisions between different objects with equal __hash__/__eq__ Regression in Django with singledispatchmethod on models #127750
• Fixes the issue of keeping object instances alive
• Adds two regression tests

There is still a cache (stored on the object instances). Quick benchmark (windows, non-pgo):

bench singledispatchmethod: Mean +- std dev: [main] 798 ns +- 64 ns -> [prx] 495 ns +- 38 ns: 1.61x faster

Benchmark hidden because not significant (1): bench singledispatchmethod slots

Geometric mean: 1.26x faster

(note that the alternative to this PR is not to keep main, but to revert #107148)

• Issue: Regression in Django with singledispatchmethod on models #127750

[dg-pb]

Check cached_property.__set_name__, it has some more stuff in it - might be needed here as well.

[eendebakpt]

Hmm. The additions there prevent something like this:

@dataclass(frozen=True)
class A:
    value: int

    @singledispatchmethod
    def dispatch(self, x):
        return id(self)

    renamed_dispatch = dispatch # allowed? if so, how should it behave

The corresponding test for the cached_property for this is

cpython/Lib/test/test_functools.py

Line 3315 in 34e840f

def test_reuse_different_names(self):

.

But on current main renaming is allowed for the singledispatchmethod.

I am not sure here what the desired behavior is (and why)

[dg-pb]

If this implementation is desirable, maybe later someone who knows more about this can comment.