requirements: update 'coverage'

Changelog:
- 4.5.2 (2018-11-12)
  (no release information whatsoever anywhere)
  https://pypi.org/project/coverage/4.5.2/

Code diff:
(unavailable)

Author: glarrain

requirements/test.txt

@@ -2,7 +2,7 @@
 -r base.txt
 
 # Required packages:
-coverage==4.5.1
+coverage==4.5.2
 flake8==3.5.0
 mypy==0.670
 #requests-mock==1.5.2