Various tox.ini improvements and cleanup

Be more explicit about when we do and don't install the project in
different testenvs by setting skip_install. This is currently
redundant, but a sensible safeguard in case the implicit behavior
from skipsdist and no usedevelop ever changes.

Use shutil.rmtree() in place of calling `rm` so that we won't need
an allowlist of executables outside the venv.

Switch to tox's {envpython} replacement macro rather than invoking
the interpreter by its assumed name, for improved portability.

Inherit {[testenv]deps} in the deps list for any testenv which needs
to extend it rather than duplicating its contents, for the sake of
maintainability.

Don't bother listing flake8 as an explicit dep in its testenv, since
the flake8-bugbear plugin will pull it in as a requirement anyway.

Update the DeprecationWarning comment about pyparsing now that the
fix is available in 3.0.8 (but still not updated where it's being
vendored).