Optional fails when default is some builtins (e.g. dict, set, list)

[gschaffner]

The documentation provides the example

schema/README.rst

Lines 271 to 277 in 09c00ed

	default can also be a callable:
	.. code:: python
	>>> from schema import Schema, Optional
	>>> Schema({Optional('data', default=dict): {}}).validate({}) == {'data': {}}
	True

This does not work, however:

>>> from schema import Schema, Optional 
>>> Schema({Optional('data', default=dict): {}}).validate({})
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/tmp/tmp.27EtQAmlOm/.venv/lib/python3.10/site-packages/schema.py", line 431, in validate
    new[default.key] = _invoke_with_optional_kwargs(default.default, **kwargs) if callable(default.default) else default.default
  File "/tmp/tmp.27EtQAmlOm/.venv/lib/python3.10/site-packages/schema.py", line 277, in _invoke_with_optional_kwargs
    s = inspect.signature(f)
  File "/usr/lib/python3.10/inspect.py", line 3247, in signature
    return Signature.from_callable(obj, follow_wrapped=follow_wrapped,
  File "/usr/lib/python3.10/inspect.py", line 2995, in from_callable
    return _signature_from_callable(obj, sigcls=cls,
  File "/usr/lib/python3.10/inspect.py", line 2529, in _signature_from_callable
    raise ValueError(
ValueError: no signature found for builtin type <class 'dict'>

I've tested and found the above error to occur with schema v0.7.5 on CPython 3.10.2, CPython 3.7.12, and PyPy3.8-7.3.7.

It looks like this regression was introduced in 958e1ad. Arguably this is due to a bug in Python, though.

Curiously: inspect.signature(list) works fine in CPython 3.6.15, but raises in 3.7.0.

For users wanting to work around this, some working options include e.g. Optional(..., default=lambda: dict()) and Optional(..., default=dict.__call__).

[gschaffner]

Duplicate of #272.