Use plpythonu in plpython tests added by commit 0b7719f74

Use plpythonu in plpython tests added by commit 0b7719f74

Commit 0b7719f74 added regression tests that spell the language name
as plpython3u.  That works on the master branch, but on v14 the
plpython tests must use the unversioned name plpythonu: the plpython3u
extension is built only in Python 3 builds, and for those builds
regress-python3-mangle.mk rewrites plpythonu to plpython3u in the test
files.  In a Python 2 build the new tests instead failed with
"language \"plpython3u\" does not exist".

Spell the language as plpythonu in the new tests, matching every other
plpython test in this branch, so they work in both Python 2 and Python
3 builds.  This is needed only in v14; later branches no longer
support Python 2 and have dropped the mangling step.

Per buildfarm member hippopotamus.