Port away from should_be

[frozencemetery]

should_be doesn't work with python-3.8 and it's unmaintained, so we need something else before fc31 testing stops being viable.

[stanislavlevin]

Hi, @frozencemetery. Some time ago I filed the identical issue #130. Since that time I build python-gssapi for my distro without should_be. If somebody interests I could open related PR.

[frozencemetery]

Unless I'm missing something, #130 is for something different: our test suite works just fine up through 3.7 as far as I can tell. (Certainly it works on fedora 31, which is python3-3.7.8-1.fc31, so...)

I am of course happy to consider pull requests to address this issue :) Please do submit them in the form of pull requests (and not attached .patch files).