TravisCI times out #46

In the course of adding py3.8 support in PR #44 the author also added mypy==0.720 which tipped the testing matrix for py36 over the 50 minute time limit.

This issue proposes changing the .travis.yml to partition the testing matrix.

How that should happen... is open discussion.

Over at vcrpy they use the following:

env:
    matrix:
        - TOX_SUFFIX="flakes"
        - TOX_SUFFIX="requests"
        - TOX_SUFFIX="httplib2"
        - TOX_SUFFIX="boto3"
        - TOX_SUFFIX="urllib3"
        - TOX_SUFFIX="tornado4"
        - TOX_SUFFIX="aiohttp"
script:
- tox -e "${TOX_SUFFIX}"

https://github.com/kevin1024/vcrpy/blob/master/.travis.yml

So we could partition by pytest versions or mypy versions.

There will have to be some careful interplay between the .travis.yml and tox.ini