Code responsible for low-level HTTP interactions.
At the moment this just contains the default HTTP backend for intessa.API, based on the urllib3 library for Python.
An extended urllib3.PoolManager with an intessa HTTP backend.
This allows you to create your own custom pool, and then use its intessa_http_backend() method as the http attribute on an API. By default, a global instance is created as DEFAULT_POOL_MANAGER and this is used as the source of the default HTTP backend function.