Bug 1559975 - fix python2 lint issues in tools/mach_commands.py r=ahal,catlee

Changes:
- add `print_function` to satisfy py2 linter

Differential Revision: https://phabricator.services.mozilla.com/D37117

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

tools/lint/py2.yml

@@ -36,7 +36,6 @@ py2:
         - tools/docs
         - tools/jprof/split-profile.py
         - tools/lint
-        - tools/mach_commands.py
         - tools/profiler
         - tools/rb
         - tools/update-packaging

tools/mach_commands.py

@@ -2,7 +2,7 @@
 # License, v. 2.0. If a copy of the MPL was not distributed with this
 # file, # You can obtain one at http://mozilla.org/MPL/2.0/.
 
-from __future__ import absolute_import, unicode_literals
+from __future__ import absolute_import, print_function, unicode_literals
 
 import sys