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/Example/NestedUnion/Test.py

@@ -4,17 +4,18 @@
 
 import flatbuffers
 from flatbuffers.compat import import_numpy
+from typing import Any
 np = import_numpy()
 
 class Test(object):
     __slots__ = ['_tab']
 
     @classmethod
-    def SizeOf(cls):
+    def SizeOf(cls) -> int:
         return 4
 
     # Test
-    def Init(self, buf, pos):
+    def Init(self, buf: bytes, pos: int):
         self._tab = flatbuffers.table.Table(buf, pos)
 
     # Test