python: add deprecation warning for APIv2
    
    See https://fedora-copr.github.io/posts/EOL-APIv1-APIv2
    
    The DeprecationWarning is displayed only when running a code with
    `python -Wall`, or in `python3 -m pytest`. Otherwise it is disabled by
    default.
    
    > DeprecationWarning - Base category for warnings about deprecated
    > features when those warnings are intended for other Python
    > developers (ignored by default, unless triggered by code in
    > __main__).
    
    Instead, FutureWarning seems to do what we want
    
    > FutureWarning - Base category for warnings about deprecated features
    > when those warnings are intended for end users of applications that
    > are written in Python.