Remove Python 3.2 from trove classifier and README

[johnthagen]

Mypy 0.4.3 dropped support for Python 3.2.

This PR also adds the generic 'Programming Language :: Python :: 3' trove classifier.

Question: Should

if sys.version_info < (3, 2, 0):
    sys.stderr.write("ERROR: You need Python 3.2 or later to use mypy.\n")
    exit(1)

Be changed to?

if sys.version_info < (3, 3, 0):
    sys.stderr.write("ERROR: You need Python 3.3 or later to use mypy.\n")
    exit(1)