Add trove classifier for PyPy support

[jdufresne]

Tested during CI since 761da6f.

[davidism]

Is there a reason beyond "users could technically filter by them in PyPI" to use these? I don't use them in any other projects, in Pallets or otherwise.

Basically, we're not going out of our way to make this a "cpython library" or a "pypy library", besides maybe a small compat entry here or there. So they don't really seem appropriate.

[jdufresne]

I find them useful because they allow potential users to know -- at a quick glance -- what Pythons are supported by a library. Sometimes when integrating a library with my own projects, I'll use this information to help decide if it is good for integration.

As the trove classifiers have a consistent form, they are also used by sites and offline scripts to analyze the changes in the Python community. Some examples:

• https://python3wos.appspot.com/
• https://hugovk.github.io/drop-python/

If you don't support PyPy as a platform, then I don't think you should add it. But PyPy is included in CI, so it appears that compatibility is desired. I think adding CPython is only useful in contrast to PyPy so as not to give the impression that the library is PyPy only.

Up to you.