refine pyyaml req

Summary:
we use the `sort_keys` argument which was not always present, see comment on related issue for details

resolves #3955

Test Plan: bk

Reviewers: dgibson, rexledesma, cdecarolis

Reviewed By: rexledesma

Differential Revision: https://dagster.phacility.com/D7176

python_modules/dagster/setup.py

@@ -61,7 +61,7 @@ def get_version() -> str:
             "click>=5.0",
             "coloredlogs>=6.1, <=14.0",
             "Jinja2",
-            "PyYAML",
+            "PyYAML>=5.1",
             # core (not explicitly expressed atm)
             "alembic>=1.2.1",
             "croniter>=0.3.34",