Add tests_require argument to setup.py.
Add tests_require argument to setup.py.

Addresses https://github.com/jensengroup/propka-3.1/pull/36#discussion_r429100526

Since the tests_require argument is deprecated, I'm also leaving the
requirements.txt file to make sure the dependencies are captured in the
future.

jensengroup/propka-3.1,jensengroup/propka