Duo-client

- Added support for Python 3.9
- Added a simple function in duo_client.Admin to fetch a single endpoint
- Added an example script using the Duo Trust Monitor Events iterator
- Added admin client methods for external password management API endpoints
- Removed extraneous Host header when running with Python 3

* Added support for billing edition and Telephony credits endpoints

- Added webauthn credential methods
- Added support for updating helpdesk message
- Improved support for multiple aliases
- Added new methods for trust monitor events
- Various bug fixes

- Added backoff and retry logic to rate limited requests
- Moved the minimum TLS version to TLS1.0. Up from SSLv3.
- Added single user directory sync to the Admin API

This is an incremental update to to the Admin API client code and examples, to support pagination of API results.

Pull in support for:
- Admin Logon factor restriction
- Username alias manipulation
- Python 3.7