MAINT - Install playwright for all of our tests

[trallard]

Ensures playwright is always available for tests following conversations in #2119 (comment)

Note, however, that the test_version_switcher_highlighting test is currently failing due to changes to the version switcher component (as part of recent a11y improvements) and due to RTD version switcher no longer being flushed into the sidebar (#2034).

I could try and add an alternative test that checks perhaps that we have at least a latest and a dev version in the version switcher?

[peytondmurray]

Tested this locally with tox run -ve compile-assets,i18n-compile,py312-sphinx61-tests. Looks like it did run the playwright tests 👍

Local testing replicated only one of these failures - the version switcher - could this be a test that should have been failing before (but wasn't, because it wasn't being run)?

FAILED tests/test_playwright.py::test_version_switcher_highlighting[chromium] - playwright._impl._errors.TimeoutError: Locator.get_attribute: Timeout 30000ms exceeded.

[trallard]

Definitely, that is the same test that I saw failing. Since that test was added the version switcher has changed a bit so nobody noticed this failing until now.