* 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
