Link python3-config to python-config

[jakirkham]

Python supplies a python-config command line tool to aid with finding headers, libraries, etc. Currently this is included in the package as python${PY_VER}-config with a link to it from python3-config. Would be nice to have python-config link to it as well. This should then mask any python-config coming from the system, which could cause issues for build systems that grab that one instead.