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/python_annotations_variable_c_runme.py

@@ -0,0 +1,24 @@
+import sys
+
+# Variable annotations for properties is only supported in python-3.6 and later (PEP 526)
+if sys.version_info[0:2] >= (3, 6):
+    from python_annotations_variable_c import *
+
+    # No SWIG __annotations__ support with -builtin or -fastproxy
+    annotations_supported = not(is_python_builtin() or is_python_fastproxy())
+
+    if annotations_supported:
+        ts = TemplateShort()
+        anno = ts.__annotations__
+        if anno != {'member_variable': 'int'}:
+            raise RuntimeError("annotations mismatch: {}".format(anno))
+
+        ts = StructWithVar()
+        anno = ts.__annotations__
+        if anno != {'member_variable': 'int'}:
+            raise RuntimeError("annotations mismatch: {}".format(anno))
+
+        ts = StructWithVarNotAnnotated()
+        if getattr(ts, "__annotations__", None) != None:
+            anno = ts.__annotations__
+            raise RuntimeError("annotations mismatch: {}".format(anno))