Use a declarative syntax to avoid mixing code and configuration.
Simplifies handling of long description and version by reducing some
boilerplate.

For details on this setuptools feature, see:
https://setuptools.readthedocs.io/en/latest/setuptools.html#configuring-setup-using-setup-cfg-files

Documents the project as Python 3 only to potential and current library
users.

As the project no longer supports Python 2, the wheel is not
"universal". From
https://wheel.readthedocs.io/en/stable/user_guide.html?highlight=universal#building-wheels:

> If your project … is expected to work on both Python 2 and 3, you will
> want to tell wheel to produce universal wheels …

* Remove py27 support

* [py27] Remove useless compatibility files

* Setup pre-commit hooks

* Run initial `tox -e lint`

* Fix package name

* Fix .travis.yml

Using pytest and tox directly is simpler and more conventional then
going through setup.py. Using setup.py installs packages as eggs where
as tox uses the more typical pip.

Overall simplifies setup.py.

Fixes #415

Helps pip decide what version of the library to install. This is
especially useful when removing support for EOL Python versions.

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.