improving python3 regression test setup

I have developed a patch to make the python3 regression test setup a bit
simpler.  Currently, we are making mangled copies of
plpython/{expected,sql} to plpython/python3/{expected,sql}, and run the
tests in plpython/python3.  This has the disadvantage that the
regression.diffs file, if any, ends up in plpython/python3, which is not
the normal location.  If we instead make the mangled copies in
plpython/{expected,sql}/python3/, we can run the tests from the normal
directory, regression.diffs ends up the normal place, and the pg_regress
invocation also becomes a lot simpler.  It's also more obvious at run
time what's going on, because the tests end up being named
"python3/something" in the test output.