not ready for py3

[safanaj]

Apparently this line https://github.com/amendlik/gitstack-pillar/blob/develop/_pillar/gitstack.py#L86
cannot works on PY3

2019-04-11 23:12:07,106 [salt.pillar      :984 ][ERROR   ][6684] Exception caught loading ext_pillar 'gitstack':
  File "/usr/lib/python3/dist-packages/salt/pillar/__init__.py", line 974, in ext_pillar
    key)
  File "/usr/lib/python3/dist-packages/salt/pillar/__init__.py", line 902, in _external_pillar_data
    *val)
  File "/var/cache/salt/master/extmods/pillar/gitstack.py", line 86, in ext_pillar
    if len(salt.utils.gitfs.GitPillar.__init__.im_func.func_code.co_varnames) > 2:

2019-04-11 23:12:07,339 [salt.pillar      :1036][CRITICAL][6684] Pillar render error: Failed to load ext_pillar gitstack: 'function' object has no attribute 'im_func'

[safanaj]

on PY3 we should use:

salt.utils.gitfs.GitPillar.__init__.__code__.co_varnames