Suggestion: Use pure-Python implementation as fallback

[taleinat]

Since there already is a pure-Python impementation used for PyPy, why not also use it as a fallback for when the C implementation isn't available, e.g. if compilation fails?

See for example the setup.py file in the simplejson project, or a simplified version in my fuzzysearch library.

Relevant especially on Windows (though that could also be solved by supplying binary wheels) but also on macOS etc.

Related to issue #1.