Do not depend on distutils

[mgedmin]

In 21d3969 I added a dependency on distutils.text_file.TextFile() because I thought that's what setuptools used to read MANIFEST.in files.

Later I realized that was wrong and that setuptools has its own code for this. I decided to stay with distutils because the stdlib moves slower, and I thought this would protect check-manifest users from breaking changes in setuptools.

However this comes at the cost of accuracy, and also people are talking about deprecating distutils: https://discuss.python.org/t/pep-632-deprecate-distutils-module/5134