"PY_SSIZE_T_CLEAN macro must be defined for '#' formats" problem present when already fixed upstream last October 29
PyQt 5.15.6 was released October 29 last year, with the fix for the problem PY_SSIZE_T_CLEAN macro must be defined for '#' formats
that is seen when running Python 3.10.
For some reason the Manjaro PyQt5 package does not appear to contain this fix, despite its version number being 5.15.6.
As far as I can tell this problem is unique to Manjaro’s PyQT5 5.15.6 package. The PyQT5 5.15.6 fix has been successfully incorporated in other distros, e.g. Debian / Ubuntu, Fedora, and very likely Arch.