mypy's strict_concatenate is deprecated

We're supposed to use `--extra-checks` instead but it doesn't have a
documented config setting. Just remove it.

Cf python/mypy#16189

Author: masklinn

pyproject.toml

@@ -86,7 +86,7 @@ warn_redundant_casts = true
 
 # these can be overridden (maybe?)
 strict_equality = true
-strict_concatenate = true
+# strict_concatenate = true
 check_untyped_defs = true
 disallow_subclassing_any = true
 disallow_untyped_decorators = true