vcs: avoid node base class knowledge of sub classes Pytype very reasonably got confused over the sub class property references in the base class. Fixed by moving base class specific parts of comparison to the sub classes and calling the base class. With .content only referenced in FileNode, there is even less need for an explicitly failing .content getter for DirNode.

# 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
.