Skip some job manager tests on python 3.7

no support for `shift` on `time_machine` fixture below python 3.8

refs #590, #596, #578

tests/extra/test_job_management.py

@@ -1,6 +1,7 @@
 import datetime
 import json
 import re
+import sys
 import textwrap
 import threading
 import time
@@ -491,6 +492,9 @@ def start_job(row, connection_provider, connection, **kwargs):
         assert set(result.status) == {"running"}
         assert set(result.backend_name) == {"foo"}
 
+    @pytest.mark.skipif(
+        sys.version_info < (3, 8), reason="`time_machine` fixture with `shift` is not available below python 3.8. #578"
+    )
     @pytest.mark.parametrize(
         ["start_time", "end_time", "end_status", "cancel_after_seconds", "expected_status"],
         [