[3.6] Bump pytest-cov from 2.7.1 to 2.8.0 (#4144)

Author: dependabot-preview[bot]

requirements/ci-wheel.txt

@@ -9,7 +9,7 @@ coverage==4.5.4
 gunicorn==19.9.0
 multidict==4.5.2
 pytest==5.2.0
-pytest-cov==2.7.1
+pytest-cov==2.8.0
 pytest-mock==1.11.0
 tox==3.13.2
 trustme==0.5.2; platform_machine=="x86_64"  # cryptography doesn't work on i686