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.

The nose project has ceased development. From their docs page:

https://nose.readthedocs.io/

> Note to Users
>
> Nose has been in maintenance mode for the past several years and will
> likely cease without a new person/team to take over maintainership.
> New projects should consider using Nose2, py.test, or just plain
> unittest/unittest2.

Simplify test infrastructure by using the stdlib unittest discover
command. One fewer dependency.

Python 2.6 support was removed in
91152df142bdde134d84ed27963cda8e6b878416.

Drop unittest2 dependency. All necessary testing features are included
in the stdlib unittest.