lint: make mypy happy

Author: dshemetov

epidatpy/_covidcast.py

@@ -397,11 +397,9 @@ def __getitem__(
             r = self._source_by_name.get(source_signal)
             assert r is not None
             return r
-        if isinstance(source_signal, tuple):
-            s = self._signals_by_key.get((source_signal[0], source_signal[1]))
-            assert s is not None
-            return s
-        raise NotImplementedError("Invalid key. Accepts either str or Tuple[str, str].")
+        s = self._signals_by_key.get((source_signal[0], source_signal[1]))
+        assert s is not None
+        return s
 
     @staticmethod
     def create(