Add support for Python variable annotations as a feature.

Both function annotations and variable annotations are turned on using the
"python:annotations" feature. Example:

  %feature("python:annotations", "c");

  struct V {
    float val;
  };

The generated code contains a variable annotation containing the C float type:

  class V(object):
      val: "float" = property(_example.V_val_get, _example.V_val_set)
      ...

Python 3.5 and earlier do not support variable annotations, so variable
annotations can be turned off with a "python:annotations:novar" feature flag.
Example turning on function annotations but not variable annotations globally:

  %feature("python:annotations", "c");
  %feature("python:annotations:novar");

or via the command line:

  -features python:annotations=c,python:annotations:novar

Closes #1951

Doc/Manual/Contents.html

@@ -1478,7 +1478,7 @@
 </ul>
 <li><a href="Python.html#Python_python3support">Python 3 Support</a>
 <ul>
-<li><a href="Python.html#Python_annotations">Function annotations</a>
+<li><a href="Python.html#Python_annotations">Python function annotations and variable annotations</a>
 <ul>
 <li><a href="Python.html#Python_annotations_c">C/C++ annotation types</a>
 </ul>