Fix CI failure with tox 4.56 by setting skip_missing_interpreters = true (#1033)

## Summary

Supersedes #1026 (Renovate update of tox 4.55.1 -> 4.56.1), whose CI is
failing.

tox 4.56.0 changed the default of `skip_missing_interpreters` back to
`false` (tox-dev/tox#3966).

The `build` job in `.github/workflows/auto-testing.yml` is a matrix
where each runner installs only a single Python version and then runs
plain `tox`, so all envs for interpreters not installed on that runner
(e.g. `py3.10` on the 3.11 runner) now fail with "could not find python
interpreter" instead of being skipped.

This PR adds `skip_missing_interpreters = true` to the `[tox]` section
of `tox.ini` to restore the previous behavior.

---------

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: mokuzon

requirements-dev.txt

@@ -1,7 +1,6 @@
 -r requirements.txt
 -r requirements-test.txt
-tox==4.55.1 ; python_version >= "3.10"
-tox==4.55.1 ; python_version < "3.10"
+tox==4.56.1
 wheel==0.47.0
 Sphinx==9.0.4 ; python_version >= "3.11"
 Sphinx==7.4.7 ; python_version < "3.11"

tox.ini

@@ -1,4 +1,8 @@
 [tox]
+# tox 4.56.0 changed the default of skip_missing_interpreters back to false
+# (https://github.com/tox-dev/tox/pull/3966). CI runners install only one
+# Python version per job, so missing interpreters must be skipped, not failed.
+skip_missing_interpreters = true
 envlist =
     py3.10,
     py3.11,