Use minimum version of pytest with bug fix for pytest-dev/pytest#3506

[stevepiercy]

We have to run tests on Windows for Python 2.7, but the current version of pytest has a bug that causes an infinite recursion. There is an open issue at pytest-dev/pytest#3506. When pytest fixes that issue, then we can change the minimum version of pytest to that. The issue is open as of pytest 3.7.3, released 2018-08-26.

[stevepiercy]

Closed by #60