Optionally generate Python type annotations (#7858)

* optionally generate type prefixes and suffixes for python code

* fix codegen error when qualified name is empty

* WIP: Python typing

* more progress towards python typing

* Further iterate on Python generated code typing

* clang-format

* Regenerate code

* add documentation for Python type annotations option

* generate code with Python type annotations

* handle forward references

* clang-format

tests/MyGame/Example2/Monster.py

@@ -29,10 +29,10 @@ class Monster(object):
         self._tab = flatbuffers.table.Table(buf, pos)
 
 def MonsterStart(builder):
-    return builder.StartObject(0)
+    builder.StartObject(0)
 
 def Start(builder):
-    return MonsterStart(builder)
+    MonsterStart(builder)
 
 def MonsterEnd(builder):
     return builder.EndObject()