Various small improvements

Remove the global variable for PYTHON_VERSION (had bugs)

Add in a check for the major version on it's own. If you are searching for
version 3.2, then the following should be acceptable to state support
  Programming Language :: Python :: 3

Add in a distinction for libraries which don't explicitly list any version
  It's quite possible they already support python 3, but the PyPi record
  just doesn't state which python versions are supported.
  This a somewhat different case from where the versions are listed, but
  the required version cannot be found.