Fix install TypeError

Activity install target used by downstream activity packaging did fail
with;

Traceback (most recent call last):
  File "setup.py", line 21, in <module>
    bundlebuilder.start()
  File "/usr/lib/python3.7/dist-packages/sugar3/activity/bundlebuilder.py", line 633, in start
    globals()['cmd_' + options.command](config, options)
  File "/usr/lib/python3.7/dist-packages/sugar3/activity/bundlebuilder.py", line 513, in cmd_install
    options.install_desktop_file)
  File "/usr/lib/python3.7/dist-packages/sugar3/activity/bundlebuilder.py", line 290, in install
    for f in self.get_files_in_git():
  File "/usr/lib/python3.7/dist-packages/sugar3/activity/bundlebuilder.py", line 209, in get_files_in_git
    stdout.strip('\n').split('\n')]
TypeError: a bytes-like object is required, not 'str'

Workaround was to hide .git directory.

Cause was subprocess pipe returning a bytes-like object in Python 3.

Regression introduced by incomplete ed46c21 ("Port to six").

Fixes https://github.com/sugarlabs/sugar-toolkit-gtk3/issues/426

src/sugar3/activity/bundlebuilder.py

@@ -206,7 +206,7 @@ class Packager(object):
             elif stdout:
                 # pylint: disable=E1103
                 git_output = [path.strip() for path in
-                              stdout.strip('\n').split('\n')]
+                              stdout.decode().strip('\n').split('\n')]
                 files = []
                 for line in git_output:
                     ignore = False