Add suggest if for sourcery (#279)

Because sourcery requires additional CI (auth) setup, we probably don't
want to turn it on automatically, even if people already have the
config.

linters/sourcery/plugin.yaml

@@ -21,6 +21,7 @@ lint:
           in_place: true
       runtime: python
       package: sourcery
+      suggest_if: never
       direct_configs:
         - .sourcery.yaml
         - sourcery.yaml