fix: arrow 'timestamp' property removed in >=v1.0.0

The missing property in newer arrow versions, caused that variables only contained references like '<bound method Arrow.timestamp of <Arrow [2023-01-15T02:16:14.627656+00:00]>>' instead the actual timestamp values.
Tests in 'tests/test_pagure_flask_dump_load_ticket.py' failed with 'ValueError: could not convert string to float'.

References
https://arrow.readthedocs.io/en/latest/releases.html
Quote: 'Calls to the timestamp property now emit a DeprecationWarning.'

arrow-py/arrow#832
Quote: 'The .timestamp property has been removed to improve compatibility with datetime. Use .int_timestamp for the same results.'

libgit2/pygit2#1122

pagure/lib/model.py

@@ -152,7 +152,10 @@ def create_default_status(session, acls=None):
 
 
 def arrow_ts(value):
-    return "%s" % arrow.get(value).timestamp
+    if hasattr(arrow, 'timestamp'):
+        return "%s" % arrow.get(value).timestamp # arrow < v1.0.0
+    else:
+        return "%s" % arrow.get(value).int_timestamp # arrow >= v1.0.0
 
 
 class AccessLevels(BASE):