DOC: Document project as Python 3 only with a trove classifier

[jdufresne]

No description provided.

[mattip]

Do we need both this line and line 43? Can you point to the "HOWTO" documentation for these specifiers?

[jdufresne]

These lines are simply metadata to be consumed by humans and scripts to help determine compatibility and other criteria when integrating third party packages from pip. No project strictly needs any classifiers, but they are useful information to those looking for the information (and for scripts). Some documentation for using classifiers exists here:

• https://pypi.org/classifiers/

Personally, I find them both useful and suggest keeping both. It doesn't require any additional maintenance and provides more information to people and scripts. Other projects, such as Django, provide both: https://pypi.org/project/Django/

[mattip]

I see that the 1.16 version retains the older classifiers, so this PR will not influence that one. It is strange that we have a Programming Language :: Python :: Implementation :: CPython when we run CI tests with PyPy, but that is a subject for another day.

[mattip]

Thanks @jdufresne