version 1.1.0 failing at _clean_text_signature with TypeError: expected string or bytes-like object

Version 1.0 has been ok. Possible related to #221

The method _clean_text_signature seems to be also receiving classes and not being able to handle them.

Added few print to debug. These are the result:

    print(sig)  # pytest.importorskip.__text_signature__
    print(type(sig))  # <class 'pytest.importorskip.__text_signature__'>
    print(start_pattern.search(sig))  # CRASH: TypeError

See the failing job: https://gitlab.com/secml/secml/-/jobs/622655243

Still investigating any possible source of the problem from our side.

Installed Sphinx modules:

    'sphinx.ext.autodoc',
    'sphinx.ext.autosummary',
    'sphinx.ext.viewcode',
    'sphinx.ext.intersphinx',
    'sphinx.ext.imgmath',
    'matplotlib.sphinxext.plot_directive',
    'numpydoc',
    'm2r',
    'nbsphinx',
    'nbsphinx_link'

Installed libraries:

numpydoc==1.1.0
Sphinx==2.4.4
sphinx-rtd-theme==0.5.0

Start at the _clean_text_signature entry point and reproduce the failure with the reported Sphinx 2.4.4 and numpydoc 1.1.0 environment. Trace why a class reaches start_pattern.search(sig), then verify that the failing documentation build no longer raises the TypeError.