Remove support for end-of-life Python 2.7

Python 2.7 is end of life. It is no longer receiving bug fixes,
including for security issues. Python 2.7 went EOL on 2020-01-01. For
additional details on support Python versions, see:

Supported: https://devguide.python.org/#status-of-python-branches
EOL: https://devguide.python.org/devcycle/#end-of-life-branches

Removing support for EOL Pythons will reduce testing and maintenance
resources while allowing the library to move towards a modern Python 3
style. Python 2.7 users can continue to use the previous version of
redis-py.

Was able to simplify the code:

- Removed redis._compat module
- Removed __future__ imports
- Removed object from class definition (all classes are new style)
- Removed long (Python 3 unified numeric types)
- Removed deprecated __nonzero__ method
- Use simpler Python 3 super() syntax
- Use unified OSError exception
- Use yield from syntax

Co-authored-by: Andy McCurdy <andy@andymccurdy.com>

flake8 catches a wider net of mistakes than pycodestyle and is more
commonly used by the larger community. For example, it catches unused
imports, a few of which existed. These have since been removed.

Two "noqa" comments were added. One ignores the _compat.py file as it
has a large amount of Python version specific code. The second is in
utils.py which intentionally does not use an import.

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

Setuptools allows using setup.cfg as a configuration file to define the
package metadata and options. This approach reduces boilerplate code in
favor of a declarative configuration. Down the road, this approach also
allows for automation through scripts and tools.

Starting with wheel 0.32.0 (2018-09-29), the "license_file" option is
deprecated.

The wheel will continue to include LICENSE, it is now included
automatically:

https://wheel.readthedocs.io/en/stable/user_guide.html#including-license-files-in-the-generated-wheel-file
https://wheel.readthedocs.io/en/stable/news.html

tox can now always be used as the test entry point and allows for
contributors to easily test the full test matrix locally.

Starting with setuptools v41.5.0 (27 Oct 2019) the `setup.py test`
command is formally deprecated. It now emits a warning on use. Prior to
this version, it use was discouraged by the setuptools developers.

Now, always use tox as the one testing entry point. It handles multiple
version of Python and Redis.

The Travis CI configuration was reworked to use tox to install
dependencies and run tests in the same manner that contributors would do
locally.

The wheel package format supports including the license file. This is
done using the [metadata] section in the setup.cfg file. For additional
information on this feature, see:

https://wheel.readthedocs.io/en/stable/index.html#including-the-license-in-the-generated-wheel-file