Add type annotations required by mypy

Author: sdeibel

coverage/sqldata.py

@@ -615,7 +615,7 @@ def touch_files(self, filenames: Iterable[str], plugin_name: Optional[str] = Non
                     # Set the tracer for this file
                     self.add_file_tracers({filename: plugin_name})
 
-    def purge_files(self, filenames, context=None):
+    def purge_files(self, filenames: Iterable[str], context: Optional[str] = None) -> None:
         """Purge any existing coverage data for the given `filenames`.
 
         If `context` is given, purge only data associated with that measurement context.

tests/test_api.py

@@ -802,7 +802,7 @@ def test_purge_filenames_context(self) -> None:
             """)
         fn2 = os.path.join(self.temp_dir, fn2)
 
-        def dummy_function():
+        def dummy_function() -> None:
             unused = 42
 
         # Start/stop since otherwise cantext