Add trove classifiers
Add trove classifiers

These help new library users know, at a quick glance, if the library is
suitable for including in a project. These values are displayed on the
PyPI project page.

Documents support for Python 3.5, 3.6, 3.7 and 3.8.

Documents support for PyPy in addition to CPython.

more-itertools/more-itertools,erikrose/more-itertools