* Update mypy to 1.6.1
* Fix mypy "Source file found twice under different module names" error
* Ignore type of intialized abstract class in tests
* Use more specific type ignore method-assign
* Type ignore for message.get_all
* Remove unused type ignore
* Add SizedBuffer type for xmlrpc.client.Transport subclass
* Add Self type for RequestHandlerClass in test
* Add type ignore for shutil.rmtree onexc handler
* Quote SizedBuffer
* Add news entry
* Remove no longer correct comment
* Update self import
* Also ignore type onerror=handler
* Update news entry
* Update news entry
The new functionality serves a use case which was not previously possible with pip configuration files, namely the situation where you have a base Python installation and want to influence the pip configuration for all derivative virtual environments *without* changing the config for all other environments on a machine (global), or for all other environment run by the same user (user). Concretely, this could be used for a centrally managed network mounted filesystem based Python installation, from which multiple users can build virtual environments and inside which a specific pip configuration is needed (e.g. an index URL).