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.'
    
    https://github.com/arrow-py/arrow/issues/832
    Quote: 'The .timestamp property has been removed to improve compatibility with datetime. Use .int_timestamp for the same results.'
    
    https://github.com/libgit2/pygit2/issues/1122