move to Python3

[skshetry]

dvc-s3-repo/docker/rpm/Dockerfile

Line 57 in 207192c

# NOTE: uploading with rpm-s3 requires python2, so we need to keep using fedora:26

rpm-s3 still depends on python2. dateutil removed support for Python2 recently (see dateutil/dateutil#1344).

I have pinned it for now in 207192c. But we should figure out a way to migrate it to Python3.