Version 0.15.0 (#1301)

Version 0.15.0 (#1301)

* Version 0.15.0

* Update CHANGELOG.md

Co-authored-by: Jamie Hewland <jamie.hewland@hpe.com>
* Escalate deprecations into removals.

* Deprecate overly verbose timeout parameter names

* Fully deprecate max_keepalive in favour of explicit max_keepalive_connections

* Fully deprecate PoolLimits in favour of Limits

* Deprecate instantiating 'Timeout' without fully explicit values

* Include deprecation notes in changelog

* Use httpcore 0.11.x

Co-authored-by: Jamie Hewland <jamie.hewland@hpe.com>