Bug 1603634 - use python3.6 to run mozharness tests in python3 r=jmaher

Changes:

Change the python version used in mozharness testing to python3.6 as that's what the `linter` docker image has, not `python3.5`.

Promote the tier of mozharness-py3 test to tier 1 and run it on all repositories to ensure python3 compatibility for future changes.

Differential Revision: https://phabricator.services.mozilla.com/D58209

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

taskcluster/ci/source-test/python.yml

@@ -147,15 +147,13 @@ mozharness:
 mozharness-py3:
     description: mozharness integration tests
     treeherder:
-        tier: 3
         symbol: py3(mh)
     run:
         using: run-task
         cache-dotcache: true
         cwd: '{checkout}/testing/mozharness'
         command: >
-            /usr/local/bin/tox -e py35-hg5.2
-    run-on-projects: ['mozilla-central', 'try']
+            /usr/local/bin/tox -e py36-hg5.2
     when:
         files-changed:
             - 'testing/mozharness/**'

testing/mozharness/tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py27-hg5.2, py35-hg5.2
+envlist = py27-hg5.2, py36-hg5.2
 
 [base]
 deps =
@@ -24,7 +24,7 @@ deps =
     {[base]deps}
     mercurial==5.2.1
 
-[testenv:py35-hg5.2]
+[testenv:py36-hg5.2]
 deps =
     {[base]deps}
     mercurial==5.2.1

tools/tryselect/selectors/fuzzy.py

@@ -46,7 +46,6 @@ TARGET_TASK_FILTERS = (
     '.*win64-aarch64-laptop.*',
     'android-hw.*',
     '.*android-geckoview-docs.*',
-    'source-test-python-mozharness-py3'
 )