Bug 1621960: Change |mach python| default from Python 2 to Python 3 r=rstewart

Depends on D77967

Differential Revision: https://phabricator.services.mozilla.com/D78181

python/mach_commands.py

@@ -56,12 +56,12 @@ class MachCommands(MachCommandBase):
 
         # Note: subprocess requires native strings in os.environ on Windows.
         append_env = {
-            b'PYTHONDONTWRITEBYTECODE': str('1'),
+            'PYTHONDONTWRITEBYTECODE': str('1'),
         }
 
         if no_virtualenv:
             python_path = sys.executable
-            append_env[b'PYTHONPATH'] = os.pathsep.join(sys.path)
+            append_env['PYTHONPATH'] = os.pathsep.join(sys.path)
         else:
             self._activate_virtualenv()
             python_path = self.virtualenv_manager.python_path