Add PyPy to the test matrix on Linux. Fixes jaraco/skeleton#63.
When rendering docs, preserve the syntax for defaults. Fixes jaraco/path#197.
Adopt furo theme for docs.
Indicate to use latest Python version (workaround for readthedocs/readthedocs.org/#9623). Requires also specifying the OS version (workaround for readthedocs/readthedocs.org#9635).