Update datadog to 0.21.0

[pyup-bot]

This PR updates datadog from 0.20.0 to 0.21.0.

Changelog

0.21.0

**Search hosts: `Infrastructure.search` is deprecated**
The `Infrastructure.search` method is deprecated in favor of the new `Hosts.search` method.

* [BUGFIX] API: Prevent exception contexts from logging URLs and credentials, [266][]
* [FEATURE] API: Add `search` and `totals` methods to the `Hosts` resource, [261][]

Links

• PyPI: https://pypi.org/project/datadog
• Changelog: https://pyup.io/changelogs/datadog/
• Homepage: https://www.datadoghq.com

[pyup-bot]

Closing this in favor of #489