fix pre-cmmit hooks version

.pre-commit-config.yaml

@@ -2,11 +2,11 @@
 
 repos:
   - repo: https://github.com/ambv/black
-    rev: stable
+    rev: 19.3b0
     hooks:
       - id: black
   - repo: https://github.com/pre-commit/pre-commit-hooks
-    rev: master
+    rev: 6d7906e131976a1ce14ab583badb734727c5da3e
     hooks:
       - id: flake8