plugin and hook calling mechanisms for Python 3

 pluggy is the crystallized core of plugin management and hook calling for
 pytest. It enables 1400+ plugins to extend and customize pytest’s default
 behaviour. Even pytest itself is composed as a set of pluggy plugins which are
 invoked in sequence according to a well defined set of protocols.
 .
 It gives users the ability to extend or modify the behaviour of a host program
 by installing a plugin for that program. The plugin code will run as part of
 normal program execution, changing or enhancing certain aspects of it.
 .
 This is the Python 3 library.