Prevent _GufeTozenizableMeta.__call__ from overwriting signature of __init__

[Yoshanuikabundi]

When the inspect module of the standard library is used to get the signature of a GufeTokenizable class, it sees that the metaclass has an implementation of __call__ and takes the signature from that instead of from the __new__ or __init__ method of the child class:

>>> from gufe import SmallMoleculeComponent
>>> import inspect
>>> inspect.signature(SmallMoleculeComponent)
<Signature (*args, **kwargs)>
>>> inspect.signature(SmallMoleculeComponent.__init__)
<Signature (self, rdkit: rdkit.Chem.rdchem.Mol, name: str = '')>

This results in unhelpful API documentation:

But also may affect the signature displayed by IDEs, the Jupyter help magic, and so on.

This PR restores the signature from __init__ or __new__ when a child class is created:

(I've taken screenshots from the GUFE docs, but this problem affects the OpenFE docs as well, and this solution fixes them too)

See https://stackoverflow.com/questions/49740290/call-from-metaclass-shadows-signature-of-init. I originally wrote a solution defining a __signature__ property on the metaclass, but this solution seems more... canonical.

[codecov]

Codecov Report

Patch coverage: 88.88% and project coverage change: -0.06% ⚠️

Comparison is base (799b5b2) 99.09% compared to head (2d5f27d) 99.04%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #216      +/-   ##
==========================================
- Coverage   99.09%   99.04%   -0.06%     
==========================================
  Files          36       36              
  Lines        1773     1782       +9     
==========================================
+ Hits         1757     1765       +8     
- Misses         16       17       +1     

Files Changed	Coverage Δ
gufe/tokenization.py	98.10% <88.88%> (-0.42%)	⬇️

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[richardjgowers]

thanks @Yoshanuikabundi this had been bugging me