ini: add support for adding extra entries to sections When adding entries to an existing section, assume that comments and empty lines *before* a section header belongs to that section. That is an approximation; not always entirely correct, but the best we can come up with.

# 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
.