Downgrade breaking dependency

Author: Scony

tox.ini

@@ -42,7 +42,7 @@ setenv =
 deps =
  mypy == 0.971
  types-pkg_resources
- types-PyYAML
+ types-PyYAML == 6.0.12.12
  types-docopt
 commands =
  mypy gdtoolkit/ tests/ --config-file tox.ini