Recent changes (in particular, use of the new "structural pattern
matching" syntax, which was introduced in Python 3.10) were already
incompatible with Python <3.10, but neither the tox settings nor
the project metadata had been updated to reflect this fact.
Fedora 35, the oldest currently supported branch of Fedora, already
ships with Python 3.10 by default, so we don't drop support for any
current Fedora releases.
