Get rid of jsonschema.compat usage

jsonschema.compat module is not a public interface anymore and
it got removed in version > 3.2.0
The compat module was a helper for py2/3 compatibility [1].
As Tempest is full py3 now, we can rework the whole
if-condition and let the timeutils to handle any wrong format.

[1] https://github.com/Julian/jsonschema/blob/v3.2.0/jsonschema/compat.py

Closes-Bug: #1946321
Change-Id: Ia78c19c06cfb712b1b8d0aff457fd2f91144dc07
diff --git a/tempest/lib/common/jsonschema_validator.py b/tempest/lib/common/jsonschema_validator.py
index 0ac757d..1618175 100644
--- a/tempest/lib/common/jsonschema_validator.py
+++ b/tempest/lib/common/jsonschema_validator.py
@@ -31,7 +31,7 @@
 @FORMAT_CHECKER.checks('iso8601-date-time')
 def _validate_datetime_format(instance):
     try:
-        if isinstance(instance, jsonschema.compat.str_types):
+        if instance is not None:
             timeutils.parse_isotime(instance)
     except ValueError:
         return False