python-swiftclient should use requests/urllib3-native retry mechanisms

[Tim Burke]

Having our own retry logic leads to confusing error messages. See https://bugs.launchpad.net/python-swiftclient/+bug/2011731, for example -- by default, users just get back

   Error downloading object '...': HTTPSConnectionPool(host='...', port=443): Max retries exceeded with url: ... (Caused by SSLError(SSLZeroReturnError(6, 'TLS/SSL connection has been closed (EOF) (_ssl.c:997)')))

One reasonable reaction to such a message is to pass an increased --retries option -- but that doesn't actually do anything! The message about retries is coming up out of urllib3, and we stick with the default max_retries from requests -- 0!

By using a more requests-native approach, our --retries would eventually get plumbed into urllib3 such that if users see that error, increasing --retries could actually help.