* Build HTTPSClientAuthHandler more similarly to how HTTPSHandler works
* Add docs for new client cert authentication
* Support older versions of python
* Simplify logic
* Initial support for client certs in urls.py
* Add an extra test
* Add a get_url test for client cert auth
* Add additional test for client cert auth, with validation and ssl mismatch
* Skip assert when http tester not available
* Update version_added for new options
* Make sure the uri module warns about using the deprecated HEADER_ arguments, so that we can actually deprecate in a future version
* Use module.deprecate
* No need to de-dupe ourselves, looks like it is done somewhere I didn't initially see
* Only add Content-Type if not specified in headers. Fixes#20046
* Update documentation to indicate body_format will not override Content-Type if specified in headers
