Makefile: introduce testpy-% target for testing with a specifc Python This makes it easy to do 'make testpy-2.4.6 TESTFLAGS="-j 16"' and the Makefile will build Python if needed and then run tests (with -j 16) with the resulting Python. You can set the environment variable HGPYTHONS to a nice location on your machine to cache the Python builds globally. If that's not set, it builds them inside build/pythons.