Python 3.6 reached its upstream EOL in September 2021, and was last
included in CentOS Stream 8 and Ubuntu 18.04 LTS (Bionic) which are
also both well past their EOL. This allows us to do some cleanup in
how bindep is packaged, since PEP 517 and its successors weren't
really well supported until versions of tools that have themselves
dropped support for 3.6.
Retain testing on ubuntu-bionic nodes for now, since there's
non-default Python 3.7 packaged on them, as the only platform we
presently have which meets that requirement for our new lower bound.
Change-Id: I0945465b63e4a8654c735786b3e3cf794c763741
5e3bf9940d