Fix pytest warning about cov (#2708)

JukkaL · gvanrossum · commit 9a1f6d621817 · 2017-01-18T09:05:15.000-08:00
Without this I get a pytest warning about cov every time I run it.
The cov plugin still gets loaded and I can still generate coverage
reports, so I assume that the coverage plugin is loaded
automatically. Not sure if we should require a more recent pytest
version.

This is what the warning looked like:

```
======================= pytest-warning summary ========================
WP1 None Module already imported so can not be re-written: pytest_cov
```
diff --git a/conftest.py b/conftest.py
@@ -1,4 +1,3 @@
 pytest_plugins = [
     'mypy.test.data',
-    'pytest_cov',
 ]

Original file line number	Diff line number	Diff line change
`@@ -1,4 +1,3 @@`
`1`	`1`	`pytest_plugins = [`
`2`	`2`	`'mypy.test.data',`
`3`		`- 'pytest_cov',`
`4`	`3`	`]`