Fix python tests requirements

```
twisted 20.3.0 has requirement attrs>=19.2.0, but you'll have attrs 18.2.0 which is incompatible
```

This patch updates python requirements to try to fix the travis CI.

Author: nfk

requirements/requirements-test.txt

@@ -1,7 +1,7 @@
 -r requirements.txt
 
-pytest==3.4.0
 coverage<4.4
-pytest-cov==2.4.0
+pytest>=3.4.0
+pytest-cov>=2.4.0
 codeclimate-test-reporter==0.2.3
-attrs>=17.4.0
+attrs>=19.2.0