Bug 1606182 - add python-test using python3 to make check r=Callek

Changes:

Fix the import statement that causes issues in python3.

Differential Revision: https://phabricator.services.mozilla.com/D58383

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

modules/libpref/test/test_generate_static_pref_list.py

@@ -10,7 +10,10 @@ import sys
 import unittest
 import yaml
 from os import path
-from StringIO import StringIO
+try:
+    from StringIO import StringIO
+except ImportError:
+    from io import StringIO
 
 sys.path.append(path.join(path.dirname(__file__), ".."))
 from init.generate_static_pref_list import generate_code

testing/testsuite-targets.mk

@@ -269,6 +269,9 @@ check::
 	@echo "Starting 'mach python-test' with -j$(cores)"
 	@$(topsrcdir)/mach --log-no-times python-test -j$(cores) --subsuite default
 	@echo "Finished 'mach python-test' successfully"
+	@echo "Starting 'mach python-test' with --python $(PYTHON3) -j$(cores)"
+	@$(topsrcdir)/mach --log-no-times python-test --python python3 -j$(cores) --subsuite default
+	@echo "Finished 'mach python-test' with py3 successfully"
 
 
 .PHONY: \