test failures on wsl

there seem to currently be at least three failures in local testing with py36:

• tests/sandboxes/mounting/mount_simple.py::test_bind_mount
  • https://gitlab.com/snippets/1892870

these seem to be due to casd api change:

• tests/artifactcache/expiry.py::test_invalid_cache_quota[1-success-None]
• tests/artifactcache/expiry.py::test_invalid_cache_quota[0-success-None]
  • https://gitlab.com/snippets/1892871