Bug 1638976 - Convert 'mach power' to run with Python 3. r=glandium

Differential Revision: https://phabricator.services.mozilla.com/D76273

mach

@@ -35,7 +35,6 @@ py2commands="
     mochitest
     mozharness
     pastebin
-    power
     prettier-format
     puppeteer-test
     python

tools/power/mach_commands.py

@@ -65,7 +65,8 @@ class MachCommands(MachCommandBase):
                                          '--show-process-coalition',
                                          '--show-process-gpu',
                                          '-n', '1',
-                                         '-i', interval])
+                                         '-i', interval],
+                                        universal_newlines=True)
 
         # When run with --show-process-coalition, |powermetrics| groups outputs
         # into process coalitions, each of which has a leader.