CI broken due to python/mypy#4693

[JelleZijlstra]

Due to python/mypy#4693, mypy now requires an install of every Python version used to run it. This breaks our CI because we run mypy_test.py without 3.3, 3.6, or 3.7 installed: https://travis-ci.org/python/typeshed/jobs/364264726.

Two solutions:

• Install all the necessary Python versions in Travis
• Make mypy support running with --python-version 3.3 if there is no 3.3. This means it would not be able to find PEP 561-style packages. This is fine for the use case of typeshed's CI.

cc @ethanhs