Since Python 2 is officially no longer supported, I think it's reasonable to target Python 3 going forward. You could tag the last version of pyexiftool supporting Python 2, and if you really want to be nice, maintain a bug fix branch for that version which could be installed from source.
As far as which version to target, I feel like I generally don't see many packages targeting anything below 3.6. But that's a subjective assessment. macOS users generally use the Homebrew packages, which provide Python 3.7+. Debian Stretch (old stable) provides 3.5. Debian Buster (stable) provides 3.7. Ubuntu 18.04LTS provides 3.6. Ubuntu 20.04LTS provides 3.8. I don't know what Windows users do.
Given these deployments, I would say 3.6 or 3.7 seem like good options. I guess it also largely depends on what built-in's you intend to use.
EDIT: For myself, I target 3.6 for library code but use it across many environments.
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