Fixes warning when running tests:

```
tests/oauth1/rfc5849/endpoints/test_base.py:63
  oauthlib/tests/oauth1/rfc5849/endpoints/test_base.py:63: DeprecationWarning: invalid escape sequence \d
    headers['Authorization'] = sub('timestamp="\d*k?"',
```

Helps pip decide what version of the library to install.

https://packaging.python.org/guides/distributing-packages-using-setuptools/#python-requires

> If your project only runs on certain Python versions, setting the
> python_requires argument to the appropriate PEP 440 version specifier
> string will prevent pip from installing the project on other Python
> versions.

https://setuptools.readthedocs.io/en/latest/setuptools.html#new-and-changed-setup-keywords

> python_requires
>
> A string corresponding to a version specifier (as defined in PEP 440)
> for the Python version, used to specify the Requires-Python defined in
> PEP 345.

Neither used by Travis CI nor by tox.ini. The mock package was out of
sync with requirements-tests.txt for Python 3 environments. Rather than
maintain this duplicate, unused list of requirements just remove it.

Remove unused 'requires' variable form setup.py