Bump pytest to 4.3.1

Pytest 4.3.1 includes the fix from

	pytest-dev/pytest#4795

Signed-off-by: Sebastiaan van Stijn <[email protected]>

Author: thaJeztah

requirements-dev.txt

@@ -2,5 +2,5 @@ coverage==4.4.2
 ddt==1.2.0
 flake8==3.5.0
 mock==3.0.5
-pytest==3.6.3
+pytest==4.3.1
 pytest-cov==2.5.1