feat: Add retry strategy with backoff of 0.5s

[tembleking]

This PR implements retries in the Python SDK.

[brackendawson]

Hi, this appears to have broken IAM authentication:

Traceback (most recent call last):
  File "/home/jenkins/workspace/Containers-Registry/sync-sysdig-alerts/vaultfiles/scripts/sysdig/sync-alerts.py", line 29, in <module>
    source_client = create_sdcclient_for_instance(source_instance)
  File "/home/jenkins/workspace/Containers-Registry/sync-sysdig-alerts/vaultfiles/scripts/sysdig/sync-alerts.py", line 25, in create_sdcclient_for_instance
    ibm_headers = IbmAuthHelper.get_headers(url, api_key, guid)
  File "/usr/local/lib/python3.7/dist-packages/sdcclient/ibm_auth_helper.py", line 15, in get_headers
    iam_token = IbmAuthHelper.__get_iam_token(url, apikey)
  File "/usr/local/lib/python3.7/dist-packages/sdcclient/ibm_auth_helper.py", line 35, in __get_iam_token
    response = self.http.post(
NameError: name 'self' is not defined

I've raised #164

Thanks,
Bracken.