issues: backout special whitespace handling This is essentially a backout of commit 32e1e0745d3c. That commit checked for whitespace at the beginning of the matched issue reference, and explicitly retained it in the resulting link text. The way this was handled is not only suboptimal, e.g. a set of 4 spaces would still be reduced to 1, but is also not actually necessary: if whitespace before the issue reference is not required, then it does not need to be specified in the issue pattern, and if it _is_ required, then a positive lookbehind assertion can be used instead.

# 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
.