Python 3.13 is the current version and was released over a month ago
(see https://devguide.python.org/versions). The dependencies we use now
support Python 3.13, most importantly `fonttools` which uses OS-specific
builds and for which compatibility got introduced in
https://github.com/fonttools/fonttools/pull/3656 and the corresponding
`cp313` wheels for all distributions are published on
https://pypi.org/project/fonttools/#files.
Moreover, we fix forgotten `npx` usage in the font tests README which
was encountered while testing this patch.
913738644a