get-toolchain should retry (with backoff) if it gets "rate limit exceeded" from github

[mithro]

Currently the following happens;

0.36s$ python ./get-toolchain.py
Traceback (most recent call last):
  File "./get-toolchain.py", line 158, in <module>
    sys.exit(main(sys.argv[1:]))
  File "./get-toolchain.py", line 113, in main
    toolchain_data = get_toolchain_data()
  File "./get-toolchain.py", line 108, in get_toolchain_data
    response = urlopen(request)
  File "/usr/local/Cellar/python@2/2.7.17_1/Frameworks/Python.framework/Versions/2.7/lib/python2.7/urllib2.py", line 154, in urlopen
    return opener.open(url, data, timeout)
  File "/usr/local/Cellar/python@2/2.7.17_1/Frameworks/Python.framework/Versions/2.7/lib/python2.7/urllib2.py", line 435, in open
    response = meth(req, response)
  File "/usr/local/Cellar/python@2/2.7.17_1/Frameworks/Python.framework/Versions/2.7/lib/python2.7/urllib2.py", line 548, in http_response
    'http', request, response, code, msg, hdrs)
  File "/usr/local/Cellar/python@2/2.7.17_1/Frameworks/Python.framework/Versions/2.7/lib/python2.7/urllib2.py", line 473, in error
    return self._call_chain(*args)
  File "/usr/local/Cellar/python@2/2.7.17_1/Frameworks/Python.framework/Versions/2.7/lib/python2.7/urllib2.py", line 407, in _call_chain
    result = func(*args)
  File "/usr/local/Cellar/python@2/2.7.17_1/Frameworks/Python.framework/Versions/2.7/lib/python2.7/urllib2.py", line 556, in http_error_default
    raise HTTPError(req.get_full_url(), code, msg, hdrs, fp)
urllib2.HTTPError: HTTP Error 403: rate limit exceeded
The command "python ./get-toolchain.py" failed and exited with 1 during .