Dropping support for Python 3.3

[medmunds]

I'm planning to stop testing Python 3.3 after the next Anymail release (v2.1).

The only version of Django that supported Python 3.3 was Django 1.8, which ended extended support on April 1, 2018.

I'm not immediately planning to drop Anymail support/testing for Django 1.8 on other Python versions, but Python 3.3 has a particularly problematic early release of the upgraded Python 3 email package. I can't imagine many people are using this combination, and tracking down oddities seems like a poor use of time. If you are using Django 1.8 on Python 3.3 with Anymail (or really anything having to do with email), you should upgrade to Python 3.4 or 3.5. Or downgrade to Python 2.7.

(I'll leave this issue open for feedback until Anymail v2.1 is ready to release, in case I've underestimated the impact.)