Upgrade process_account_events.py from boto to boto3

[data-sync-user]

Tokenserver’s process_account_events.py relies on the older boto library for accessing FxA’s account events via SQS. In the long term we should migrate away from SQS entirely: https://mozilla-hub.atlassian.net/browse/SYNC-4211 but until that happens (and lacking any timeline for it) there’s reasons to migrate off the original boto to boto3:

• boto is fully deprecated and no longer receives updates
• boto is no longer compatible with modern Python versions (seemingly has issues with >= Python 3.12)
• the script’s usage of boto is very small (only a few lines of code)

This is basically a reopening of the original issue from about a year ago: https://github.com/mozilla-services/syncstorage-rs/issues/1532

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