Fix python tests requirements

pytest 3.4.0 depends on an old attrs version which is not compatible
with the last version of twisted.

```
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 +1,5 @@
 -r requirements.txt
 
-pytest==3.4.0
-coverage<4.4
-pytest-cov==2.4.0
+pytest>=5.4.3
+pytest-cov>=2.10.0
 codeclimate-test-reporter==0.2.3
-attrs>=17.4.0