Enable dependabot for pip

[janosh]

Auto-updates requirements{,-optional}.txt monthly.

@shyuep Decided against using the req.txt files in test.yml workflow. We're installing purely from setup.py there.

pymatgen/.github/workflows/test.yml

Line 56 in b950dc1

python -m pip install -e '.[dev,optional]'

By installing unpinned packages in CI, we catch breaking changes in dependencies sooner. Once something breaks CI, we can compare with requirements.txt for a "last working version" to see which update was the culprit.

[coveralls]

Coverage remained the same at 0.0% when pulling b950dc1 on pip-dependabot into c3f589d on master.