test: bump pytest minimum version Recent test case changes broke compatibility with pytest < 3.0. FYI, the ~= version spec is a shorthand (PEP 440) used here to select "3.0 or any later 3.* release". We could probably use this more often.

# 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
.