deps: update pytest from 4.1.1 to 4.2.0

Update pytest from 4.1.1 to 4.2.0 (#26)

requirements.txt

@@ -1,4 +1,4 @@
 mock==2.0.0
 pylint==1.9.3
-pytest==4.1.1
+pytest==4.2.0
 snapshottest==0.5.0