Add HookImpl.__repr__

This is helpful when debugging hooks being called.

Example:

> <HookImpl plugin_name='testmon', plugin_file='/path/to/plugin.py'>

Author: blueyed

pluggy/hooks.py

@@ -315,3 +315,9 @@ def __init__(self, plugin, plugin_name, function, hook_impl_opts):
         self.opts = hook_impl_opts
         self.plugin_name = plugin_name
         self.__dict__.update(hook_impl_opts)
+
+    def __repr__(self):
+        return "<HookImpl plugin_name=%r, plugin_file=%r>" % (
+            self.plugin_name,
+            self.plugin.__file__,
+        )