Update coverage from 5.3 to 5.4 (#55)

Author: pyup-bot

requirements_dev.txt

@@ -4,7 +4,7 @@ wheel==0.36.2
 watchdog==0.10.3
 flake8==3.8.4
 tox==3.20.1
-coverage==5.3
+coverage==5.4
 Sphinx==3.3.0
 twine==3.3.0