[TODO][good first issue] Update packaging metadata to clarify support for Python 3.7+

[webknjaz]

So apparently, the wheel published on PyPI is tagged as py2.py3 for which the label Python 2 shows up on the web page @ https://pypi.org/project/scriv/1.5.1/#files.

The Trove classifiers only list Python versions starting with 3.7, though. And that's the lowest version found in CI and various linters like MyPy.

This of course, can lead to some confusion both for the users and the installer software. Here's what I propose.

Action items

• remove the [wheel] ([bdist_wheel]) section from setup.cfg — this will result in the .whl files having just the py3 tag in their names
• add python_requires = >= 3.7 to setup.cfg under [options] — this will make sure a Require-Python is included in the packaging metadata, allowing the installers to know what runtimes are supported; if done after Transition into pyproject.toml #117, an equivalent setting should be added to pyproject.toml

[nedbat]

Fixed in commit b87175c. Thanks for the instructions!