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.

setup.py

@@ -57,5 +57,6 @@ See http://propka.org/ for the PROPKA web server.
         },
       zip_safe=True,
       python_requires='>=3.5',
+      tests_require=["pandas", "pytest"],
       test_suite="tests",
 )