config: abort early if the environment doesn't allow Python to pass Unicode strings to the file system layer Dulwich passes unicode paths to the file system layer and it will thus not work if run with LC_ALL=C ... which is quite common for services. Also try to detect different cases and give helpful error messages about what environment variable could be changed to what.

# 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
.