Pin mypy and drop type annotations which makes mypy unhappy

This also:
- drops spurious copyright headers to those files if present.
- drops a no longer relevant type ignore
- add another type ignore (to fix later)

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__)

swh/indexer/origin_head.py

@@ -47,7 +47,7 @@ class OriginHeadIndexer(OriginIndexer[Dict]):
             self, "_try_get_%s_head" % visit_status.type, self._try_get_head_generic
         )
 
-        rev_id = method(snapshot.branches)
+        rev_id = method(snapshot.branches)  # type: ignore
         if rev_id is not None:
             return [{"origin_url": origin_url, "revision_id": rev_id,}]
 

swh/indexer/storage/model.py

@@ -52,7 +52,7 @@ class BaseRow:
 
     @classmethod
     def from_dict(cls: Type[TSelf], d) -> TSelf:
-        return cls(**d)  # type: ignore
+        return cls(**d)
 
     def unique_key(self) -> Dict:
         obj = self

tox.ini

@@ -35,7 +35,7 @@ commands =
 extras =
   testing
 deps =
-  mypy
+  mypy==0.920
 commands =
   mypy swh