Upgrade to Python 3.9

[nathanielrindlaub]

We are currently stuck on Python 3.7 because in Lambda environments >3.7 the Perl executable we load via a Lambda Layer gets stored in /opt/bin/perl, where as previously I think it was stored in /usr/bin/perl.

This is problematic because the exiftool executable directs the computer to use perl in the latter location in the first line of the script (#!/usr/bin/perl -w).

The fix is likely to change that line to #!/opt/bin/perl -w in a step in the Dockerfile.