testsuite: Ubuntu 14.04 is End of Life

[hmaarrfk]

Description

https://wiki.ubuntu.com/Releases

I'm not too sure how this affects the wheel builder. If we drop 14.04 from our test matrix, I think we need to update the manylinux standard to a newer one

https://www.python.org/dev/peps/pep-0571/

xref: #4082

[hmaarrfk]

https://github.com/matthew-brett/multibuild/issues/238

[grlee77]

closing this as stale. I don't think we still use Ubuntu 14.04 anywhere (GitHub actions appear to be using either 18.04 or 20.04)