Use in-memory database to speed up tests

This also avoids problems when test tables are changed,
in which case we would need to drop and recreate them.

graphene_sqlalchemy/tests/test_query.py

@@ -11,7 +11,7 @@
 from ..utils import sort_argument_for_model, sort_enum_for_model
 from .models import Article, Base, Editor, Hairkind, Pet, Reporter
 
-db = create_engine("sqlite:///test_sqlalchemy.sqlite3")
+db = create_engine("sqlite://")  # use in-memory database
 
 
 @pytest.yield_fixture(scope="function")