[Python] Fixed the issue with nested unions relying on InitFromBuf. (#7576)

* feat: Fixed the issue with nested unions relying on InitFromBuf.
Problem: Issue #7569
Nested Unions were broken with the introduction of parsing buffers with an initial encoding offset.

Fix:
Revert the InitFromBuf method to the previous version and introduction of InitFromPackedBuf that allows
users to read types from packed buffers applying the offset automatically.

Test:
Added in TestNestedUnionTables to test the encoding and decoding ability using a nested table with a
union field.

* fix: Uncommented generate code command

tests/MyGame/Example/NestedUnion/Any.py

@@ -0,0 +1,20 @@
+# automatically generated by the FlatBuffers compiler, do not modify
+
+# namespace: NestedUnion
+
+class Any(object):
+    NONE = 0
+    Vec3 = 1
+    TestSimpleTableWithEnum = 2
+
+def AnyCreator(unionType, table):
+    from flatbuffers.table import Table
+    if not isinstance(table, Table):
+        return None
+    if unionType == Any().Vec3:
+        import MyGame.Example.NestedUnion.Vec3
+        return MyGame.Example.NestedUnion.Vec3.Vec3T.InitFromBuf(table.Bytes, table.Pos)
+    if unionType == Any().TestSimpleTableWithEnum:
+        import MyGame.Example.NestedUnion.TestSimpleTableWithEnum
+        return MyGame.Example.NestedUnion.TestSimpleTableWithEnum.TestSimpleTableWithEnumT.InitFromBuf(table.Bytes, table.Pos)
+    return None