Update coverage from 4.5.1 to 5.3 (#21)

Co-authored-by: Jason Hu <[email protected]>

Author: pyup-bot

requirements_dev.txt

@@ -4,7 +4,7 @@ wheel==0.35.1
 watchdog==0.10.3
 flake8==3.5.0
 tox==3.20.1
-coverage==4.5.1
+coverage==5.3
 Sphinx==1.7.1
 twine==1.10.0