Newer version of mypy fixes buggy error when trying to update only boto3 stubs.
However it brings new checks and starts to yell when we index into
cusror.fetchone without checking for None first. So this introduces a wrapper
to simplify quering for scalar values. I tried to use cursor_factory connection
argument but without success. There can be a better way to do that,
but this looks the simplest
Use timestamp->LSN mapping instead of file modification time.
Fix 'latest_gc_cutoff_lsn' - set it to the minimum of pitr_cutoff and gc_cutoff.
Add new test: test_pitr_gc
