Bug 1562935 - Update Mozlog version to 4.2.0 in wptrunner requirement…

…s r=ato

The mozlog version needed to be updated in the wptrunner requirement.txt in
order for Bug 1555014 to successfully merge upstream.

Differential Revision: https://phabricator.services.mozilla.com/D36628

--HG--
extra : moz-landing-system : lando

testing/web-platform/tests/tools/wptrunner/requirements.txt

@@ -1,6 +1,6 @@
 html5lib==1.0.1
 mozinfo==1.1.0
-mozlog==4.1
+mozlog==4.2.0
 mozdebug==0.1.1
 pillow==6.0.0
 urllib3[secure]==1.25.3