fix: Include tox in test-requirements.txt

The contribution instructions direct new code contributors to run test using
`tox`. However, `tox` is not included in the `test-requirements.txt` file,
requiring an extra step for new contributors to install `tox` manually.

This commit adds `tox` to the `test-requirements.txt` file so that new code
contributors automatically get it when they install the test requirements.

Author: thehale

test-requirements.txt

@@ -3,3 +3,4 @@ pytest-cov
 python-openid;python_version<="2.7"
 python3-openid;python_version>="3.0"
 pytest-mock>=1.10
+tox