py3: add PY23() macro to switch string literal depending on python version
I have no better idea to work around the bytes-unicode divergence of
Py_BuildValue(). Maybe we can write a code transformer for C extensions? :)
#
# Avoid interference from actual test env:
unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL