[http_client] remove requests exception contexts

[yannmh]

On Python 3, remove requests exception contexts before raise a new exception to avoid
accidentally leaking URLs and credentials (API and application keys).

Traceback (most recent call last): 
File "/usr/local/lib/python3.6/site-packages/datadog/api/http_client.py", line 73, in request 
result.raise_for_status() 
File "/usr/local/lib/python3.6/site-packages/requests/models.py", line 935, in raise_for_status 
raise HTTPError(http_error_msg, response=self) 
requests.exceptions.HTTPError: 500 Server Error: Internal Server Error for url: https://app.datadoghq.com/api/v1/downtime?api_key=<redacted>&application_key=<redacted>

During handling of the above exception, another exception occurred:

Traceback (most recent call last): 
[...]
File "/usr/local/lib/python3.6/site-packages/datadog/api/resources.py", line 147, in get_all 
return APIClient.submit('GET', cls._class_url, **params) 
File "/usr/local/lib/python3.6/site-packages/datadog/api/api_client.py", line 129, in submit 
proxies=_proxies, verify=_cacert 
File "/usr/local/lib/python3.6/site-packages/datadog/api/http_client.py", line 84, in request 
raise HTTPError(e.response.status_code, result.reason) 
datadog.api.exceptions.HTTPError: Datadog returned a bad HTTP response code: 500 - Internal Server Error. Please try again later. If the problem persists, please contact support@datadoghq.com" 

[nmuesch]

Hey, thanks for taking care of this! I'm wondering if down the line we want to come up with a more universal scrubber for a more generic set of cases. But this looks like it will get the job done 🎉