Drop legacy distutils fallback in setup.py

[jdufresne]

distutils is not recommended for use and unnecessary for modern Python
environments. Use only setuptools instead. From
https://docs.python.org/3/library/distutils.html:

Most Python users will not want to use this module directly, but
instead use the cross-version tools maintained by the Python Packaging
Authority. In particular, setuptools is an enhanced alternative to
distutils ...

The recommended pip installer runs all setup.py scripts with
setuptools, even if the script itself only imports distutils. Refer to
the Python Packaging User Guide for more information.

[GhostofGoes]

Looks like it's "vendored" now in setuptools, so this seems safe to drop.

[McSinyx]

I don't agree: on many GNU/Linux distros, setuptools is separately packaged from Python and pip, thus if we take this approach, while appdirs is still installable by pip, local setup.py might not run.

[jdufresne]

Is that really an issue in practice? In my experience:

1. The default system Python has setuptools installed by default
2. Linux users use their system package manager to install system libraries
3. Linux users use a virtualenv with pip to install packages for local testing

[pradyunsg]

Nudging on this PR for merge.