WIP: Better help for -m option and python2/3 interop
Created by: wpostma
Apologies for mixing two concerns in one MR/branch here... Still getting the hang of this.
Two changes here:
- Reinstate pytz when python3 only module fails to import
- Friendlier (in my humble opinion) help for -m
-m MIN_AGE, --min-age MIN_AGE
Minimum age for artifacts to be deleted;
-m '30 days' : 30 days
-m '8 hours' : 8 hours
-m 60 : 60 seconds