Docs build fails due to __getattr__ intercepting dunder attributes during Sphinx introspection

[Jiya873]

While working on PR #616, I ran into a docs build failure on Read the Docs that seems unrelated to the changes in that PR.

During Sphinx autosummary generation, class introspection triggers access to __mro__, which is intercepted by pymc_extras.prior.__getattr__. This leads to:

UnsupportedDistributionError: PyMC doesn't have a distribution of name '__mro__'

This appears to happen because __getattr__ attempts to resolve unknown attributes as PyMC distributions, including Python dunder attributes that are needed for Sphinx introspection.

Error Log

File ".../site-packages/sphinx/util/inspect.py", line 317, in isstaticmethod
    for basecls in getattr(cls, '__mro__', [cls]):
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File ".../site-packages/pymc_extras/prior.py", line 1582, in __getattr__
    _get_pymc_distribution(name)
File ".../site-packages/pymc_extras/prior.py", line 246, in _get_pymc_distribution
    raise UnsupportedDistributionError(f"PyMC doesn't have a distribution of name {name!r}")
pymc_extras.prior.UnsupportedDistributionError: PyMC doesn't have a distribution of name '__mro__'

[Jiya873]

Right now __getattr__ attempts to resolve all unknown attribute names as PyMC distributions. During Sphinx introspection, this includes Python dunder attributes such as __mro__, which leads to the docs build failure. A possible way to avoid this would be to have __getattr__ immediately raise AttributeError for dunder attributes, allowing normal Python introspection to proceed while preserving the current dynamic lookup behavior.