Added poetry integration.

Description:
* Fixed github actions;
* Removed requirements.txt;
* Fixed CONTRIBUTING.md;
* Fixed black and flake8.

Signed-off-by: Pavel <win10@list.ru>

ormar/models/mixins/save_mixin.py

@@ -222,7 +222,7 @@ class SavePrepareMixin(RelationMixin, AliasMixin):
 
     @staticmethod
     async def _upsert_through_model(
-        instance: "Model", previous_model: "Model", relation_field: "ForeignKeyField",
+        instance: "Model", previous_model: "Model", relation_field: "ForeignKeyField"
     ) -> None:
         """
         Upsert through model for m2m relation.