style: use monospace for 'formatted-fixed' class The 'formatted-fixed' class is applied to PR comments, PR descriptions and repository descriptions. It already preserves whitespace via 'whitespace: pre-wrap', but the font is not monospace. As a result, diagrams or other ASCII art do not display well. Use monospace on this class to fix that.

# 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
.