Support signatures with PEP 649 deferred annotations

[Otto-AA]

Closes #463

Python 3.14 supports deferred annotation loading. However, the inspect.signature raises an Error when trying to compute non-existing lazy-loaded annotations (which happens with if TYPE_CHECKING imports)

Instead of:

{orig_name}.__signature__ = _mutmut_signature({mangled_name}__mutmut_orig)

we now do:

_mutmut_copy_signature({orig_name}, {mangled_name}__mutmut_orig)

# ...

def _mutmut_copy_signature(trampoline, original_method):
    if _mutmut_sys.version_info >= (3, 14):
        # PEP 649 introduced deferred loading for annotations
        # When some_method.__annotations__ is accessed, Python uses some_method.__annotate__(format) to compute it
        # By copying the original __annotate__ method, we provide get the original annotations
        trampoline.__annotate__ = original_method.__annotate__
    else:
        # On Python 3.13 and earlier, __annotations__ can be accessed immediately
        trampoline.__annotations__ = original_method.__annotations__

    try:
        trampoline.__signature__ = _mutmut_inspect.signature(original_method)
    except NameError:
        # Also, because of PEP 649, it can happen that we cannot eagerly evaluate the signature
        # In this case, fall back to stringifying the signature (which could cause different behaviour with runtime introspection)
        trampoline.__signature__ = _mutmut_inspect.signature(original_method, annotation_format=_mutmut_inspect.Format.STRING)

I think this works in following cases:

• Python 3.13 inspect.get_annotations
• Python 3.13 inspect.signature
• Python 3.14 inspect.get_annotations
• Python 3.14 inspect.signature for eagerly available signatures
• Python 3.14 inspect.signature with deferred annotations (when mutmut cannot eagerly compute signature(...) it now stringifies the signature, which could break some introspection)

According to the docs, get_annotations is the best practice to read annotations, so I think it's okay that we're not fully supporting the inspect.signature for deferred annotations case. I've created #465 for future improvements.