Drop all Python 3.2 support

[gvanrossum]

We already don't support running mypy itself on Python 3.2. We've now decided that even supporting 3.2 as a target platform is too much work to maintain, so we're going to drop it completely.

See also python/typeshed#1145.

[ambv]

The latest release of typing already dropped 3.2 support. Good to see this happening here as well! :)