Re-introduce support for Python < 2.7.9
Re-introduce support for Python < 2.7.9

42bfde0082f5c26bc94b94f62fcffcbcc14969b2 introduced a change that relied
on an Exception class (ssl.SSLWantReadError) present only in modern Python
versions. This changes that to work exactly the same way, only in a way
that works for older Python versions also.

Uberi/python-slackclient,slackapi/python-slackclient,sqlbyme/python-slackclient,slackapi/python-slackclient,miohtama/python-slackclient,piranha/python-slackclient,slackhq/python-slackclient,nosman/python-slackclient,mathieu-wang/python-slackclient,slackapi/python-slackclient,Tolsadus/python-slackclient