Do not build wheel for python 2, restrict to >=3.7

[rly]

Motivation

Released wheels are named hdmf-3.0.1-py2.py3-none-any.whl which means it can be installed on Python 2, but Python >=3.7 is required. This PR fixes the issue.

Also fixes a minor CI typo where the wrong tox environments were being run for a job.

Checklist

• Did you update CHANGELOG.md with your changes?
• Have you checked our Contributing document?
• Have you ensured the PR clearly describes the problem and the solution?
• Is your contribution compliant with our coding style? This can be checked running flake8 from the source directory.
• Have you checked to ensure that there aren't other open Pull Requests for the same change?
• Have you included the relevant issue number using "Fix #XXX" notation where XXX is the issue number? By including "Fix #XXX" you allow GitHub to close issue #XXX when the PR is merged.