Revision 1869853

Support building with SWIG 4 on Python 3.x

* build/ac-macros/swig.m4
  (SVN_FIND_SWIG): Allow building with SWIG 4+, and add -modern option
    when Python 3 and SWIG 3.x are detected.

* subversion/bindings/swig/include/proxy.py
    Use _get_instance_attr and _set_instance_attr.

* subversion/bindings/swig/include/proxy.swg
  (_get_instance_attr): New function to get an instance attribute
    without metadata for new-style and old-style classes.
  (_set_instance_attr): New function to set an instance attribute for
    new-style and old-style classes.

* subversion/bindings/swig/INSTALL
  (BUILDING SWIG BINDINGS FOR SVN ON UNIX, Step 1): Update supported
    SWIG versions for Python 3 bindings (remove the note that SWIG 4+
    is not supported).

Patch by: Jun Omae <jun66j5{_AT_}gmail.com>

Review by: brane
           futatuki
           julianfoad