Update twython/twython.py

More http to https conversions for security.

I left the part at line 292 "if endpoint.startswith('http://'):
            url = endpoint"
as it is because the original implementer might have something else in mind, but it is supposed to be defaulting to https with the following code change:

if endpoint.startswith('http://'):
            url = endpoint.replace('http://', 'https://', 1)