Fix curry/rcurry/autocurry crashing on unbound built-in methods

[binggao1230]

Problem

curry(), rcurry(), and autocurry() crash with ValueError when passed
an unbound method of a built-in type such as str.endswith, str.startswith,
or list.append:

>>> from funcy import rcurry
>>> has_suffix = rcurry(str.endswith)
# ValueError: Unable to introspect str.endswith() arguments

These functions are method_descriptor objects. inspect.signature() cannot
introspect them on CPython, and the fallback in get_spec() raised
unconditionally. Workaround was to pass n= explicitly (rcurry(str.endswith, 2)),
but that shouldn't be required. Reported in #108.

Fix

method_descriptor objects always expose __objclass__ (the class they belong
to). When signature() raises and __objclass__ is set, fall back to a
two-positional-argument spec — one for the implicit self and one for the
primary method argument. This covers the common use-case:

has_suffix = rcurry(str.endswith)   # no n= needed
has_suffix('.py')('file.py')        # True

starts_with_hello = rcurry(str.startswith)
starts_with_hello('Hello')('Hello World')  # True

Methods that need a different arity can still use the explicit n= form.

Changes

• funcy/_inspect.py: handle __objclass__ in the signature() except branch
• tests/test_funcs.py: add test_curry_builtin_method covering the new behaviour

All 206 tests pass.

[Suor]

What about other methods, which have more than one argument besides self? This code will try to make them 2 args curry too.

Also, comments are two verbose. Please prepare a proper PR, not just what coding agent does without much thinking.

[binggao1230]

You're right. This fallback is not a proper fix: it treats every non-introspectable builtin method descriptor as if n=2, which is not generally true, and even the motivating str.endswith case has optional start/end parameters. I also saw the older issue discussion recommending explicit n rather than partial builtin-method introspection.

I'm closing this PR rather than pushing a narrow/incorrect heuristic further.