pygmentsutils: separate from util2 __get_lem and __get_index_filenames has nothing in common with the rest of util2, neither with respects to implementation nor usage, and belongs in a dedicated module. Also, they are clearly not actually private, so shouldn't be named with leading underscores.

# 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
.