Issue 50337 - Replace exec() with setattr()
    
    Description: _constants.py uses exec() a lot to define module
    global variables. That's rather slow and not very elegant.
    
    Get the current module object with sys.modules[__name__] and
    then use setattr() instead.
    
    https://pagure.io/389-ds-base/issue/50337
    
    Reviewed by: mreynolds (Thanks!)