* Drop Python 3.4 support.

* Exclude Python 3.4 as a supported runtime.

* Update contributing.rst

* Update supported Python versions in README file.

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.