Pin mypy and drop type annotations which makes mypy unhappy

This also drops spurious copyright headers to those files if present.

Related to T3812

swh/__init__.py

 from pkgutil import extend_path
-from typing import List
 
-__path__: List[str] = extend_path(__path__, __name__)
+__path__ = extend_path(__path__, __name__)

tox.ini

@@ -39,7 +39,7 @@ commands =
 extras =
   testing
 deps =
-  mypy
+  mypy==0.920
 commands =
   mypy swh
 

[Phabricator Migration user]

Some references in the commit message have been migrated:

• T3812 is now swh/infra/ci-cd/swh-jenkins-jobs#3812 (closed)