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

Examples/test-suite/python/Makefile.in

@@ -49,6 +49,7 @@ CPP_TEST_CASES += \
 	primitive_types \
 	python_abstractbase \
 	python_annotations_c \
+	python_annotations_variable_c \
 	python_append \
 	python_builtin \
 	python_destructor_exception \