setup: bump TurboGears minimum version to 2.4 Avoid failures starting with test_api_wrong_key: E File ".../build/minimum-dependency-versions-venv/lib/python3.7/site-packages/tg/request_local.py", line 37, in languages_best_match E items = [i for i, q in sorted(al._parsed, key=lambda iq: -iq[1])] E TypeError: 'NoneType' object is not iterable

# 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
.