sslutil: drop support for Python prior to 3.7 There's also a block of code around line 47 related to `ssl.HAS_TLSv1` to determine the supported protocols that references "Python 3.7", but I'm not altering that because the commit referenced there wasn't landed until just prior to the 3.9 release, and I'm not sure what flavors of py38 might not have a backport. Avoid de-indenting for now for a clearer text diff.