Only depend on mypy.api

[dmtucker]

Fixes #6

I think we should strongly discourage such use of interfaces internal to mypy. The only public APIs are the command line and the mypy.api module.
python/mypy#4681 (comment)

[dmtucker]

I hit the same Python 3.3 failure as @petr-muller in #7:
https://travis-ci.org/dbader/pytest-mypy/jobs/350216845

[petr-muller]

Yup, this looks even better!

[dmtucker]

Another data point for dropping Python 3.3:

| python_version | download_count |
| -------------- | -------------- |
| 3.6            |          9,940 |
| None           |          5,010 |
| 3.5            |            941 |
| 3.4            |            443 |
| 1.17           |            267 |
| 2.7            |            157 |
| 3.7            |             62 |
| 3.3            |              4 |

There have been very few 3.3 downloads in the last year.
(Produced with pypinfo --days 365 pytest-mypy pyversion)

[dbader]

Thanks! LGTM :-) If anybody helps with maintaining & possibly growing this project ping me and I'll add you as a collaborator. I've got too much on my plate right now and I'm not doing a good job at supporting this project.

[dmtucker]

@dbader I'd be interested (more for maintenance than dev, but hey, who knows).

#9 is another option.

[dbader]

Hey @dmtucker, just sent you an invite to add you as a collaborator :-)