Helps pip decide what version of the library to install. Include this
information in the README as well. These versions were already
documented as trove classifiers.

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.

Fixes the following warning when Python warnings are enabled during
tests:

    setup.py:9: ResourceWarning: unclosed file <_io.TextIOWrapper name='README.rst' mode='r' encoding='UTF-8'>
      return open(os.path.join(os.path.dirname(__file__), "README.rst")).read()

To enable warnings, use the pass the -Walways argument to Python:

https://docs.python.org/3/using/cmdline.html#cmdoption-w

Python 2.6 and 3.3 are end of life. They are no longer receiving bug
fixes, including for security issues. Python 2.6 went EOL on 2013-10-29
and 3.3 on 2017-09-29. For additional details on supported Python
versions, see:

https://devguide.python.org/#status-of-python-branches

Removing support for EOL Pythons will reduce testing and maintenance
resources.

Using pypinfo, here are the download statistics for Unidecode over the
last 30 days, showing minimal 3.3 & 2.6 installs.

$ pypinfo --percent unidecode pyversion

| python_version | percent | download_count |
| -------------- | ------- | -------------- |
| 2.7            |  66.07% |        244,263 |
| 3.6            |  19.69% |         72,777 |
| 3.5            |   8.27% |         30,585 |
| 3.4            |   5.59% |         20,663 |
| 3.7            |   0.29% |          1,062 |
| 2.6            |   0.06% |            210 |
| 3.8            |   0.02% |             72 |
| 3.3            |   0.01% |             44 |
| 3.2            |   0.00% |              6 |
| None           |   0.00% |              2 |

Traditionally Python Unidecode followed the versioning scheme used by the
original Perl module. This doesn't make sense any more because:

1) Perl Text::Unidecode itself switched to a different versioning scheme long ago
http://search.cpan.org/~sburke/Text-Unidecode-1.30/

2) Python Unidecode has now sufficiently diverged from Perl version so that
matched version numbers are no longer useful,

3) setuptools (>= 8) began mangling version numbers, yielding confusion because
Unidecode 0.04.x now sometimes appears as 0.4.x
https://github.com/pypa/setuptools/issues/302