PyAPI: Use annotations for RNA definitions

- Logical use of fields since they define type information.
- Avoids using ordered-dict metaclass.

Properties using regular assignments will print a warning and load,
however the order is undefined.

release/scripts/modules/bpy_extras/io_utils.py

@@ -52,25 +52,19 @@ def _check_axis_conversion(op):
 
 
 class ExportHelper:
-    filepath = StringProperty(
+    filepath: StringProperty(
         name="File Path",
         description="Filepath used for exporting the file",
         maxlen=1024,
         subtype='FILE_PATH',
     )
-    check_existing = BoolProperty(
+    check_existing: BoolProperty(
         name="Check Existing",
         description="Check and warn on overwriting existing files",
         default=True,
         options={'HIDDEN'},
     )
 
-    # needed for mix-ins
-    order = [
-        "filepath",
-        "check_existing",
-    ]
-
     # subclasses can override with decorator
     # True == use ext, False == no ext, None == do nothing.
     check_extension = True
@@ -112,18 +106,13 @@ class ExportHelper:
 
 
 class ImportHelper:
-    filepath = StringProperty(
+    filepath: StringProperty(
         name="File Path",
         description="Filepath used for importing the file",
         maxlen=1024,
         subtype='FILE_PATH',
     )
 
-    # needed for mix-ins
-    order = [
-        "filepath",
-    ]
-
     def invoke(self, context, event):
         context.window_manager.fileselect_add(self)
         return {'RUNNING_MODAL'}
@@ -173,11 +162,6 @@ def orientation_helper_factory(name, axis_forward='Y', axis_up='Z'):
         update=_update_axis_up,
     )
 
-    members["order"] = [
-        "axis_forward",
-        "axis_up",
-    ]
-
     return type(name, (object,), members)