PL/Python: Improve Python 3 regression test setup

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.