ImportError: cannot import name _thread

I just installed aws-shell on Mac OS X Yosemite (10.10.5) and got the following error on first execution:

```
$ aws-shell
Traceback (most recent call last):
File "/usr/local/bin/aws-shell", line 9, in
load_entry_point('aws-shell==0.1.1', 'console_scripts', 'aws-shell')()
File "/System/Library/Frameworks/Python.framework/Versions/2.7/Extras/lib/python/pkg_resources.py", line 357, in load_entry_point
return get_distribution(dist).load_entry_point(group, name)
File "/System/Library/Frameworks/Python.framework/Versions/2.7/Extras/lib/python/pkg_resources.py", line 2394, in load_entry_point
return ep.load()
File "/System/Library/Frameworks/Python.framework/Versions/2.7/Extras/lib/python/pkg_resources.py", line 2108, in load
entry = __import__(self.module_name, globals(),globals(), ['__name__'])
File "/Library/Python/2.7/site-packages/awsshell/__init__.py", line 7, in
from awsshell import shellcomplete
File "/Library/Python/2.7/site-packages/awsshell/shellcomplete.py", line 15, in
import botocore.session
File "/Library/Python/2.7/site-packages/botocore/session.py", line 26, in
import botocore.credentials
File "/Library/Python/2.7/site-packages/botocore/credentials.py", line 22, in
from dateutil.parser import parse
File "/Library/Python/2.7/site-packages/dateutil/parser.py", line 43, in
from . import tz
File "/Library/Python/2.7/site-packages/dateutil/tz/__init__.py", line 1, in
from .tz import *
File "/Library/Python/2.7/site-packages/dateutil/tz/tz.py", line 23, in
from ._common import tzname_in_python2, _tzinfo, _total_seconds
File "/Library/Python/2.7/site-packages/dateutil/tz/_common.py", line 2, in
from six.moves import _thread
ImportError: cannot import name _thread
```

I solved by following the advice of [Matplotlib issue on OS X (“ImportError: cannot import name _thread”)](http://stackoverflow.com/questions/27630114/matplotlib-issue-on-os-x-importerror-cannot-import-name-thread) and changing the versions of `python-dateutil`.

```
sudo pip uninstall python-dateutil
sudo pip install python-dateutil==2.2
```

I'm not well versed in Python, but this looks related to the version of `six` and I wanted to post this issue to see if that was the right fix and it should be added to the README, or if it is something that should be fixed.

Start at the /usr/local/bin/aws-shell entry point and trace the imports through awsshell/__init__.py and awsshell/shellcomplete.py, reproducing the Python 2.7 failure on the reported macOS setup. Determine whether the python-dateutil and six versions are the cause; done means confirming a supported dependency fix or documenting the required version in the README.

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