pyproject.toml: Set minimum Python version to 3.9
Now that we no longer need to support Debian Buster, we can set the
minimum Python version to 3.9, which is what is available in Debian
Bullseye.
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Now that we no longer need to support Debian Buster, we can set the
minimum Python version to 3.9, which is what is available in Debian
Bullseye.