Add tosa serializer to .mypy.ini

As suggested in a previous discussion the tosa serializer should go
into .mypy.ini for a project wide ignore on missing imports instead of
doing inline annotations in every file.

Change-Id: I3ea00d0ff586d0f68a2c0fc557a2296cd03f3b04

Author: perheld

.mypy.ini

@@ -77,6 +77,9 @@ ignore_missing_imports = True
 [mypy-ruamel]
 ignore_missing_imports = True
 
+[mypy-serializer.*]
+ignore_missing_imports = True
+
 [mypy-setuptools.*]
 ignore_missing_imports = True