Adhere to the standard for dropping Python versions
Maintainers from several scientific Python projects are defining a standard to for all projects to drop support of old Python versions in a synchronized way. The standard is defined in this PR: https://github.com/numpy/numpy/pull/14086

I think pandas is already quite synchronized with the rest of the ecosystem, and I think the timeline of the standard matches what we have in mind in the short term (stopping support of Python 3.5 in the next release).

I create this issue to have a discussion in case anyone has comments. And I propose that if there is agreement, we adhere to the standard formally.