hooks: use /usr/bin/env only when needed The use of /usr/bin/env is only needed for relative arguments (or to pass variables in the environment, which we don't do). It is thus not needed in case the Python interpreter for Git hooks is known with absolute path, as introduced in 5e501b6ee639.

# 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
.