tox.ini: tox4 compatibility

[jayaddison]

Ideally, the goal here is to update tox.ini so that it's backwards-compatible with tox v3, while also allowing us to upgrade to tox v4 in the nearish future.

Relates to #698.

[jayaddison]

Note: there are notes from tox upstream to indicate that pip flags should be considered deprecated, and all pip configuration settings should use the supported PIP_-prefixed environment variables as alternatives.

I'm sorta on the fence about whether that's a good idea - environment variables aren't displayed in process tree lists (ps output, for example), and the use of --only-binary=lxml is potentially kinda important here, and not good to hide unnecessarily.

In addition, there's no specific test coverage in pip for the PIP_ONLY_BINARY environment variable. It's probably covered implicitly, and the documentation suggests that it should work - but test coverage is generally a stronger guarantee.

However: duplicating the pip install_command isn't exactly long-term maintenance-free either. Especially if it becomes unavailable from tox v5 onwards, as the maintainers there indicate.

[jayaddison]

I think f5e3f1d is probably worth reverting, since this is intended to (continue to) support tox v3 environments for a while.