[RFC] NEP 29 adoption (drop <3.6 in next release)

[qwhelan]

With Python 2 reaching EOL on 1/1/2020, bottleneck will also be dropping support for Python 2.7.

With the recent adoption of NEP 29, numpy has adopted a formal version upgrade cadence. Given bottleneck's tight compatibility with numpy, it likely makes sense for us to adopt NEP 29 as well. The relevant section is the following timeline:

Date | Python | NumPy
Jan 07, 2020 | 3.6+ | 1.15+
Jun 23, 2020 | 3.7+ | 1.15+
Jul 23, 2020 | 3.7+ | 1.16+
Jan 13, 2021 | 3.7+ | 1.17+
Jul 26, 2021 | 3.7+ | 1.18+
Dec 26, 2021 | 3.8+ | 1.18+
Apr 14, 2023 | 3.9+ | 1.18+

This would imply dropping 3.5 support in the next release as well. We would not be doing anything to actively inhibit use on 3.5, just not actively testing or seeking to fix issues that solely exhibit there.

Please reach out in this thread if this would inhibit your use of bottleneck. If there is no general objection, I will adopt NEP 29 around 1/1/2020.

[qwhelan]

Adopted in #291