Convert hooks.varnames() to inspect.signature() on Python 3

Avoids a possible deprecation warning with inspect.getfullargspec()

Fixes pytest-dev#209

src/pluggy/hooks.py

@@ -124,18 +124,6 @@ def normalize_hookimpl_opts(opts):
     opts.setdefault("optionalhook", False)
 
 
-if hasattr(inspect, "getfullargspec"):
-
-    def _getargspec(func):
-        return inspect.getfullargspec(func)
-
-
-else:
-
-    def _getargspec(func):
-        return inspect.getargspec(func)
-
-
 _PYPY3 = hasattr(sys, "pypy_version_info") and sys.version_info.major == 3
 
 
@@ -163,22 +151,41 @@ def varnames(func):
         except Exception:
             return ()
 
-    try:  # func MUST be a function or method here or we won't parse any args
-        spec = _getargspec(func)
-    except TypeError:
-        return (), ()
+    # func MUST be a function or method here or we won't parse any args
+    # this is what we return if it is not
+    non_function = (), ()
 
-    args, defaults = tuple(spec.args), spec.defaults
-    if defaults:
-        index = -len(defaults)
-        args, defaults = args[:index], tuple(args[index:])
+    if hasattr(inspect, "signature"):
+        try:
+            signature = inspect.signature(func)
+        except ValueError:
+            return non_function
+
+        empty = inspect.Parameter.empty
+        variable = (inspect.Parameter.VAR_POSITIONAL, inspect.Parameter.VAR_KEYWORD)
+        parameters = signature.parameters.values()
+        parameters = tuple(p for p in parameters if p.kind not in variable)
+        args = tuple(p.name for p in parameters if p.default is empty)
+        defaults = tuple(p.name for p in parameters if p.default is not empty)
+        mangled_self = inspect.ismethod(func)
     else:
-        defaults = ()
+        try:
+            spec = inspect.getargspec(func)
+        except TypeError:
+            return non_function
+
+        args, defaults = tuple(spec.args), spec.defaults
+        if defaults:
+            index = -len(defaults)
+            args, defaults = args[:index], tuple(args[index:])
+        else:
+            defaults = ()
+        mangled_self = False
 
     # strip any implicit instance arg
     # pypy3 uses "obj" instead of "self" for default dunder methods
     implicit_names = ("self",) if not _PYPY3 else ("self", "obj")
-    if args:
+    if args and not mangled_self:
         if inspect.ismethod(func) or (
             "." in getattr(func, "__qualname__", ()) and args[0] in implicit_names
         ):