Unpin coverage again

The upstream issue [1] has been fixed and release in 6.4.

[1] coveragepy/coveragepy#1310

Author: QuLogic

requirements/testing/all.txt

@@ -1,7 +1,7 @@
 # pip requirements for all the CI builds
 
 certifi
-coverage<6.3
+coverage
 psutil
 pytest!=4.6.0,!=5.4.0
 pytest-cov