Bug 1605502: upgrade to Mercurial 5.2.1 in mozharness r=tomprince

I noticed the version of `hg` used by mozharness tests is rather
old. This commit upgrades to version 5.2.1 (the latest).

Differential Revision: https://phabricator.services.mozilla.com/D58055

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

taskcluster/ci/source-test/python.yml

@@ -139,7 +139,7 @@ mozharness:
         cache-dotcache: true
         cwd: '{checkout}/testing/mozharness'
         command: >
-            /usr/local/bin/tox -e py27-hg4.3
+            /usr/local/bin/tox -e py27-hg5.2
     when:
         files-changed:
             - 'testing/mozharness/**'
@@ -154,7 +154,7 @@ mozharness-py3:
         cache-dotcache: true
         cwd: '{checkout}/testing/mozharness'
         command: >
-            /usr/local/bin/tox -e py35-hg4.3
+            /usr/local/bin/tox -e py35-hg5.2
     run-on-projects: ['mozilla-central', 'try']
     when:
         files-changed:

testing/mozharness/tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py27-hg4.3, py35-hg4.3
+envlist = py27-hg5.2, py35-hg5.2
 
 [base]
 deps =
@@ -19,15 +19,15 @@ setenv =
 commands =
     coverage run --source configs,mozharness,scripts --branch {envbindir}/nosetests -v --with-xunit --rednose --force-color {posargs}
 
-[testenv:py27-hg4.3]
+[testenv:py27-hg5.2]
 deps =
     {[base]deps}
-    mercurial==4.3.1
+    mercurial==5.2.1
 
-[testenv:py35-hg4.3]
+[testenv:py35-hg5.2]
 deps =
     {[base]deps}
-    mercurial==4.3.1
+    mercurial==5.2.1
 
 [testenv:py27-coveralls]
 deps=