Add an option to drop Python 2 support.

[icemac]

See zopefoundation/importchecker#5 as an example where this is needed.

[mgedmin]

I would be a happier person if you accepted my suggested refactoring to avoid double negatives, but I won't veto the change if you don't want to do that.

[sallner]

I sketched here a possible solution to specify a minimum version and then support all version above that. This would also help with future python versions and projects which do not have Python 3.6 support as they were ported later. This would occasionally save some test builds.

[icemac]

@sallner wrote:

I sketched here a possible solution to specify a minimum version and then support all version above that. This would also help with future python versions and projects which do not have Python 3.6 support as they were ported later. This would occasionally save some test builds.

I looked at your PR but decided to keep it here more lightweight as the majority of the repositories here at zopefoundation should run on the same Python versions.