3147: Move fake Python HTTP response generation to arvados_testutil.

Other tests can use this functionality.
3147: Add RetryLoop to the Python SDK.

This provides a general-purpose mechanism for us to retry all kinds of
operations.
3147: Add check_http_response_success to Python SDK.

Other parts of the SDK need to end loops based on the result of an
HTTP request.  This function puts that logic in one place.
3147: Remove old Keep signing support from Python SDK.

Per Tom.
3147: Add retry support to Python SDK's KeepClient.
3147: Fix unwanted integer division in KeepClient logs.
Merge branch '3147-pysdk-retries'

Closes #3147.