What SWIG calls "modern" classes are supported by Python 2.3 and up
which means they're supported by all the Python versions we aim to
support in 4.0.0.
Conflicts:
Source/Modules/python.cxx
This is a cherry-pick and merge from the patch in #1261
Fixes nondynamic mode when an instance variable is set with the same
name as a class variable in a class derived from a SWIG proxy class.
The python_nondynamic testcase now works using -modern and testcase
enhanced slightly too.
Fixes#718
