Added poetry integration.

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

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

tests/test_model_definition/test_saving_nullable_fields.py

@@ -35,7 +35,7 @@ class SecondaryModel(ormar.Model):
     id: int = ormar.Integer(primary_key=True)
     name: str = ormar.String(max_length=100)
     primary_model: PrimaryModel = ormar.ForeignKey(
-        PrimaryModel, related_name="secondary_models",
+        PrimaryModel, related_name="secondary_models"
     )