[mod_python] Dropping of support for non threaded Python.

For reasons unknown, I am having a go at fixing:

  http://issues.apache.org/jira/browse/MODPYTHON-217
  http://issues.apache.org/jira/browse/MODPYTHON-177

One of the problems in making these changes and in the maintainability
of the mod_python code base is that it still tries to support a Python
installation that has deliberately not had thread support built in.

>From what I have seen it has now been many years since any
distribution has deliberately disabled threading in Python. Last
platform to do it were some of the BSD platforms, but these days they
build with threaded Python as far as I know.

Consequently, I would like to drop the code which supports a non
threaded Python installation. This would get rid of a whole bunch of
#if conditionals and code variants and make making changes to Python a
lot easier. As is, don't know that the unthreaded code has even been
tested for a release since 3.1.X days and thus it may not even work
anyway.

Because of this and nature of changes being made, would also like to
bump version for mod_python in subversion up to 3.4.X as it isn't
going to be a trivial bug fix relative to 3.3.1.

Comments.

Graham