Add toxenv for py38

[ericwb]

According to the tox docs, the latest python version for toxenv is 3.7. See https://tox.readthedocs.io/en/latest/example/basic.html#a-simple-tox-ini-default-environments

Now that Python 3.7 has been officially released and 3.8 is available, it would be great to add py38 to the list of supported test environments.

For now, I'm using the workaround of "py" and running it with Python 3.8 so the venv inherits.