sysconfig currently calculates various formats of the Python version number by chopping up the sys.version string. This has a FIXME by it in the code, because the the format of sys.version is not guaranteed.

With this patch, the config variables 'py_version', 'py_version_short' and 'py_version_nodot' are instead generated from sys.version_info, which has a specified structure:
https://docs.python.org/3/library/sys.html#sys.version_info

One piece of information is lost by this change: after a pre-release, a + is added to the version string - e.g. '3.5.0b4+' means an unreleased version somewhere after 3.5.0b4. I can't find any structured representation of this information, so 'py_version' no longer contains it. I'm not sure whether it matters: I can't find anything using the 'py_version' config variable.