Not sure if this helps with recent certificate issues, but we might as well
keep this modern.
Note that certifi no longer claims py2 support, and there's a PR to add it back
in. Py2 support was dropped in 2020.04.05.2 (which predates what's being
updated here). None of the *.py files have changed since the 2020.6.20 release,
and I was able to call certifi.where() in both a virtualenv and with
hg debugshell from an MSI install with this version, so we shouldn't be any
worse off than before.