Bump required pytest version to 4.6

[diazona]

We have started getting an error with pytest>=4,<4.6 when setuptools>=71 (or typeguard>=4.2) is also installed, due to a conflict with older pytest and setuptools' vendored version of typeguard. Out of various ways to resolve this conflict, this is the only one that doesn't involve putting an upper bound on the version of some package.

It should still be technically possible to use older versions of pytest along with setuptools<71, if someone really needs to do that, but there isn't any way to express that dependency constraint, and I'm not going to go to any trouble to support long-obsolete versions of packages.

Testing notes

• If I add typeguard<4.2 to the test requirements in tox.ini, then I run into Failure under pytest-6.2.5 agronholm/typeguard#343
• If I add setuptools<71 then everything works (since it doesn't require any version of typeguard at all)
• If I change to requiring pytest>=6.2, then again everything works
• It turns out I can go as far back as pytest>=4.6 and still everything works; it looks like there was a change to how and when plugins are loaded in 4.6 that affects this

Hence I'm bumping to 4.6 in this PR.

Closes #97

[diazona]

P.S. I'll make a separate PR after this one to support pytest 8.