Max retries for all connections

The client should be more robust against connection errors like:

ConnectionError('Connection aborted')
RemoteDisconnected('Remote end closed connection without response')

From the author of requests:

If Python sends a request at roughly the same time as the server closes the
session, then the server will send a RST (as the session is closed). Python
receives this RST on what it thought was a valid session and throws an
error.

This can be handled by setting max_retries in a requests.adapters.HTTPAdapter class (the requests default is 0) which will retry on a ConnectionError.

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Note that we are manually retrying uploads only (3 times). Using the general retry would be helpful for all calls.