caucase.utils: Fix support for timezone-naive-only cryptography versions

Use UTC but timezone-naive datetime everywhere when cryptography does not
expose the timezone-aware version of its date attributes.
Fixes support for Python 2.7 with cryptography 3.3.2 (at least).

caucase/utils.py

@@ -652,30 +652,31 @@ def _getAuthorityKeyIdentifier(cert):
     x509.AuthorityKeyIdentifier,
   ).value.key_identifier
 
-try:
-  _UTC = datetime.timezone.utc
-except AttributeError: # pragma: no cover
-  # BBB
-  _ZERO_TIMEDELTA = datetime.timedelta(0)
-  class _UTC(datetime.tzinfo):
-    """UTC"""
+if hasattr(x509.Certificate, 'not_valid_after_utc'): # pragma: no cover
+  try:
+    _UTC = datetime.timezone.utc
+  except AttributeError:
+    # BBB
+    _ZERO_TIMEDELTA = datetime.timedelta(0)
+    class _UTC(datetime.tzinfo):
+      """UTC"""
 
-    def utcoffset(self, dt):
-      return _ZERO_TIMEDELTA
+      def utcoffset(self, dt):
+        return _ZERO_TIMEDELTA
 
-    def tzname(self, dt):
-      return "UTC"
+      def tzname(self, dt):
+        return "UTC"
 
-    def dst(self, dt):
-      return _ZERO_TIMEDELTA
+      def dst(self, dt):
+        return _ZERO_TIMEDELTA
+    _UTC = _UTC()
 
-def utcnow():
-  """
-  Get a datetime instance of the current time in UTC.
-  """
-  return datetime.datetime.now(_UTC)
+  def utcnow():
+    """
+    Get a datetime instance of the current time in UTC.
+    """
+    return datetime.datetime.now(_UTC)
 
-if hasattr(x509.Certificate, 'not_valid_after_utc'): # pragma: no cover
   def getNotValidAfter(obj):
     """
     Return the not_valid_after date from given cyrptography object.
@@ -707,6 +708,15 @@ if hasattr(x509.Certificate, 'not_valid_after_utc'): # pragma: no cover
     return obj.revocation_date_utc
 else: # pragma: no cover
   # BBB
+
+  _UTC = None
+
+  def utcnow():
+    """
+    Get a datetime instance of the current time in UTC.
+    """
+    return datetime.datetime.utcnow()
+
   def getNotValidAfter(obj):
     """
     Return the not_valid_after date from given cyrptography object.