messaging: add requirements.txt file
    
    Use it (and __requires__ variable) in setup.py and on readthedocs
    builders correctly, per warning:
    
    WARNING: autodoc: failed to import class 'schema.BuildChrootEnded' from
    module 'copr_messaging'; the following exception was raised:
    No module named 'copr_common'
    WARNING: autodoc: failed to import class 'schema.BuildChrootStarted'
    from module 'copr_messaging'; the following exception was raised:
    No module named 'fedora_messaging'