util.toml: new robust TOML lib selector / wrapper

Jakub Ružičkarequested to merge
toml into master

python3-toml package is marked as obsolete in EL 10 and Python 3.11 provides builtin TOML loading module tomllib.

apkg.util.toml selects the best available TOML lib for load and dump while only raising error when TOML functionality is actually used when no suitable module is available.

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