vcs: remove unused code and test of it Neither git nor mercurial nodes have a _mimetype attribute. Only test_nodes.test_mimetype monkey patches it and that to me seems just wrong. Thus, remove that part and fix the test with some usefull assertions. Found by pylint.

# requirements.txt file for use as "pip install -r requirements.txt" as a
# readthedocs compatible alternative to "pip install -e ." which is a working
# alternative to "setup.py develop" which doesn't work with Mercurial 3.7
.