Fix beets.util.inspect for Python 3

Under the original compatibility shim we weren't correctly inclusing
`self` in the argument list for bound methods.

beets/util/inspect.py

@@ -16,35 +16,11 @@
 from __future__ import division, absolute_import, print_function
 
 import inspect
-from collections import namedtuple
 
 from six import PY2
 
 
-ArgSpec = namedtuple('ArgSpec', 'args varargs keywords defaults')
-
-
 def getargspec(func):
     if PY2:
         return inspect.getargspec(func)
-
-    sig = inspect.signature(func)
-    args = [
-        p.name for p in sig.parameters.values()
-        if p.kind == inspect.Parameter.POSITIONAL_OR_KEYWORD
-    ]
-    varargs = [
-        p.name for p in sig.parameters.values()
-        if p.kind == inspect.Parameter.VAR_POSITIONAL
-    ]
-    varargs = varargs[0] if varargs else None
-    varkw = [
-        p.name for p in sig.parameters.values()
-        if p.kind == inspect.Parameter.VAR_KEYWORD
-    ]
-    varkw = varkw[0] if varkw else None
-    defaults = tuple(p.default for p in sig.parameters.values()
-                     if p.kind == inspect.Parameter.POSITIONAL_OR_KEYWORD
-                     and p.default is not p.empty) or None
-
-    return ArgSpec(args, varargs, varkw, defaults)
+    return inspect.getfullargspec(func)

beetsplug/bpd/__init__.py

@@ -750,7 +750,7 @@ class Command(object):
                            u'unknown command "{}"'.format(self.name))
         func = getattr(conn.server, func_name)
         argspec = inspect.getargspec(func)
-        max_args = len(argspec.args) - 1
+        max_args = len(argspec.args) - 2
         min_args = max_args
         if argspec.defaults:
             min_args -= len(argspec.defaults)