setup: drop support for Python 2.6 It would be a a stepping stone for the migration to Python 3 to only support Python 2.7. Even though we don't make any big changes now, it might allow us to remove some workarounds or use some new forward-compatible features. Mercurial dropped support for Python 2.6 2 years ago.

# 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
.