swig-py: In 'from svn.core import *', actually import all libsvn_subr public
symbols (such as SVN_VER_MAJOR).

* subversion/bindings/swig/python/svn/core.py
  (all): Set dynamically, in a manner consistent with the other svn/*.py
    modules.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1507860 13f79535-47bb-0310-9956-ffa450edef68