restore docs/requirements.txt to fix readthedocs build
restore docs/requirements.txt to fix readthedocs build

Restores from 1214414b527babd675bedb7d7625469cf4d72272, was deleted in 351f46e85ab59330d078f8dda9c2bcd33f3d76c9

pydanny/dj-stripe,dj-stripe/dj-stripe,pydanny/dj-stripe,kavdev/dj-stripe,kavdev/dj-stripe,dj-stripe/dj-stripe