Use inspect.signature() for Python3 and up

`inspect.getargspec()` is soon to be deprecated in favor of `inspect.signature()`.

`inspect.signature` was not backported for Python2, so we will keep using `inspect.getargspec()` for that.

Resolves #541

praw/decorators.py

@@ -24,11 +24,11 @@
 from __future__ import print_function, unicode_literals
 
 import decorator
-import inspect
 import six
 import sys
 from functools import wraps
 from praw.decorator_helpers import _get_captcha, _is_mod_of_all
+from praw.internal import _make_func_args
 from praw import errors
 from warnings import simplefilter, warn
 
@@ -48,7 +48,7 @@ def alias_function(function, class_name):
     """
     @wraps(function)
     def wrapped(self, *args, **kwargs):
-        func_args = inspect.getargspec(function).args
+        func_args = _make_func_args(function)
         if 'subreddit' in func_args and func_args.index('subreddit') != 1:
             # Only happens for search
             kwargs['subreddit'] = self
@@ -164,9 +164,9 @@ def require_captcha(function, *args, **kwargs):
                 # *args currently contains a None where the captcha answer
                 # needs to go. If we put the captcha in the **kwargs,
                 # we get a TypeError for having two values of the same param.
-                func_args = inspect.getargspec(function)
-                if 'captcha' in func_args.args:
-                    captcha_index = func_args.args.index('captcha')
+                func_args = _make_func_args(function)
+                if 'captcha' in func_args:
+                    captcha_index = func_args.index('captcha')
                     args = list(args)
                     args[captcha_index] = captcha_answer
                 else:

praw/internal.py

@@ -19,6 +19,7 @@
 """
 
 from __future__ import print_function, unicode_literals
+import inspect
 import re
 import six
 import sys
@@ -75,6 +76,18 @@ def _sorted(self, *args, **kwargs):
     return _sorted
 
 
+def _make_func_args(function):
+    if six.PY3 and not hasattr(sys, 'pypy_version_info'):
+        # CPython3 uses inspect.signature(), not inspect.getargspec()
+        # see #551 and #541 for more info
+        func_items = inspect.signature(function).parameters.items()
+        func_args = [name for name, param in func_items
+                     if param.kind == param.POSITIONAL_OR_KEYWORD]
+    else:
+        func_args = inspect.getargspec(function).args
+    return func_args
+
+
 def _modify_relationship(relationship, unlink=False, is_sub=False):
     """Return a function for relationship modification.