* Use six instead of urllib2, for python 3 compat
* Open the certificate file using binary mode
On python3, os.write requires 'bytes'. Also avoid
using a too broad exception, since the issue was hard
to spot due to it.
* Do not add the header User-agent if not set
Python3 module do raise a exception if a header is
not a string-like object, and the default value is None.
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