re-vendor pluggy to 0.3.1 release and also fix the README

_pytest/vendored_packages/README.md

@@ -6,7 +6,7 @@ package, please see [this issue](https://github.com/pytest-dev/pytest/issues/944
 To update the current version, execute:
 
 ```
-$ pip install pluggy==<version> --no-compile --target=_pytest/vendored_packages
+$ pip install -U pluggy==<version> --no-compile --target=_pytest/vendored_packages
 ```
 
 And commit the modified files. The `pluggy-<version>.dist-info` directory 

_pytest/vendored_packages/pluggy.py

@@ -40,7 +40,7 @@ Pluggy currently consists of functionality for:
   is possible to register plugins for which a hook specification is not yet
   known and validate all hooks when the system is in a more referentially
   consistent state.  Setting an "optionalhook" attribution to a hook
-  implementation will avoid PluginValidationError's if a specifcation
+  implementation will avoid PluginValidationError's if a specification
   is missing.  This allows to have optional integration between plugins.
 
 - a "hook" relay object from which you can launch 1:N calls to
@@ -67,7 +67,7 @@ Pluggy currently consists of functionality for:
 import sys
 import inspect
 
-__version__ = '0.3.0'
+__version__ = '0.3.1'
 __all__ = ["PluginManager", "PluginValidationError",
            "HookspecMarker", "HookimplMarker"]
 
@@ -763,6 +763,13 @@ class PluginValidationError(Exception):
     """ plugin failed validation. """
 
 
+if hasattr(inspect, 'signature'):
+    def _formatdef(func):
+        return "%s%s" % (
+            func.__name__,
+            str(inspect.signature(func))
+        )
+else:
     def _formatdef(func):
         return "%s%s" % (
             func.__name__,