dev_requirements.txt: bump minimum pytest versions to a working set Not all combinations of pytest-related packages seem to work. Trial-and-error lead to these combinations of minimum versions. Add a minimum bound to pytest-benchmark and pytest-localserver to help pip in figuring out a suitable combination with pytest, and also to allow testing minimum combinations in the future. Drop pytest-runner, which should not be needed unless when running tests directly from setup.py.

# 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
.