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/ArrayStruct.py

@@ -4,17 +4,19 @@
 
 import flatbuffers
 from flatbuffers.compat import import_numpy
+from typing import Any
+from .MyGame.Example.NestedStruct import NestedStruct
 np = import_numpy()
 
 class ArrayStruct(object):
     __slots__ = ['_tab']
 
     @classmethod
-    def SizeOf(cls):
+    def SizeOf(cls) -> int:
         return 160
 
     # ArrayStruct
-    def Init(self, buf, pos):
+    def Init(self, buf: bytes, pos: int):
         self._tab = flatbuffers.table.Table(buf, pos)
 
     # ArrayStruct
@@ -33,28 +35,27 @@ class ArrayStruct(object):
         return self._tab.GetArrayAsNumpy(flatbuffers.number_types.Int32Flags, self._tab.Pos + 4, self.BLength())
 
     # ArrayStruct
-    def BLength(self):
+    def BLength(self) -> int:
         return 15
 
     # ArrayStruct
-    def BIsNone(self):
+    def BIsNone(self) -> bool:
         return False
 
     # ArrayStruct
     def C(self): return self._tab.Get(flatbuffers.number_types.Int8Flags, self._tab.Pos + flatbuffers.number_types.UOffsetTFlags.py_type(64))
     # ArrayStruct
-    def D(self, i: int):
-        from MyGame.Example.NestedStruct import NestedStruct
+    def D(self, i: int) -> NestedStruct:
         obj = NestedStruct()
         obj.Init(self._tab.Bytes, self._tab.Pos + 72 + i * 32)
         return obj
 
     # ArrayStruct
-    def DLength(self):
+    def DLength(self) -> int:
         return 2
 
     # ArrayStruct
-    def DIsNone(self):
+    def DIsNone(self) -> bool:
         return False
 
     # ArrayStruct
@@ -73,11 +74,11 @@ class ArrayStruct(object):
         return self._tab.GetArrayAsNumpy(flatbuffers.number_types.Int64Flags, self._tab.Pos + 144, self.FLength())
 
     # ArrayStruct
-    def FLength(self):
+    def FLength(self) -> int:
         return 2
 
     # ArrayStruct
-    def FIsNone(self):
+    def FIsNone(self) -> bool:
         return False