Python 3.7 support - deprecation of classes in the collections module

Change the base classes in the pyabc.i file to use the
collections.abc module instead of collections due to the deprecation
of the classes in the collections module in Python 3.7.

CHANGES.current

@@ -7,6 +7,22 @@ the issue number to the end of the URL: https://github.com/swig/swig/issues/
 Version 4.0.0 (in progress)
 ===========================
 
+2018-06-12: wsfulton
+            [Python] The %pythonabc feature in pyabc.i now uses base classes
+              collections.abc.MutableSequence
+              collections.abc.MutableMapping
+              collections.abc.MutableSet
+            instead of
+              collections.MutableSequence
+              collections.MutableMapping
+              collections.MutableSet
+            as the latter are deprecated in Python 3.7 and are due to be removed in Python 3.8.
+            The classes in collections.abc.* are available from Python 3.3 onwards. If you
+            require support for Python 3.2, then copy the pyabc.i file and modify by removing
+            the few instances of the .abc sub-module.
+
+            *** POTENTIAL INCOMPATIBILITY ***
+
 2018-06-12: olly,wsfulton
             [Python] #701 Remove support for Python versions < 2.7 and 3.0 and 3.1.