changelog: make the table a re-usable template element This moves the changelog table to a separate template file, without further changes, except the necessary renaming to make it fully parameterized without accessing c directly. Based on work by Dominik Ruf.

# 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
.