use the '__iter__' attribute to determine if something is iterable vs a volatile error message

[k0s]

oauth2 uses the error string 'is not iterable' after try to cast to a list:

https://github.com/simplegeo/python-oauth2/blob/master/oauth2/__init__.py#L135

This is a fragile way of determining if something is an iterable or not. Instead, the 'iter' attribute can be checked for.

As a case in point, for python 2.4 the error message is:

list(12345)
Traceback (most recent call last):
File "", line 1, in ?
TypeError: iteration over non-sequence

See https://bugzilla.mozilla.org/show_bug.cgi?id=789972#c12 which is the motivation for this pull request

[k0s]

I've also added a commit that imports the sha function from the sha module for the legacy case:

try:
from hashlib import sha1
sha = sha1
except ImportError:
# hashlib was added in Python 2.5
import sha

https://github.com/simplegeo/python-oauth2/blob/master/oauth2/__init__.py#L46

Since we call 'sha' in oauth2, this results in

File "C:\Users\cltbld\test\talos\oauth2__init__.py", line 491, in sign_reques
t
digested = sha(self.body).digest()
TypeError: 'module' object is not callable

See https://bugzilla.mozilla.org/show_bug.cgi?id=789972#c16

[joestump]

@k0s Thanks for the PR! Can you split out 2.4 fixes from the error fix? Also, needs tests.