Use getfullargspec under the scenes for py3 to stop DeprecationWarning (#2864)

Use getfullargspec under the scenes for py3 to stop DeprecationWarning. 

Closes #2862

scrapy/utils/python.py

@@ -10,7 +10,6 @@ import errno
 import six
 from functools import partial, wraps
 import sys
-import time
 
 from scrapy.utils.decorators import deprecated
 
@@ -198,10 +197,30 @@ def binary_is_text(data):
     return all(c not in _BINARYCHARS for c in data)
 
 
+def _getargspec_py23(func):
+    """_getargspec_py23(function) -> named tuple ArgSpec(args, varargs, keywords,
+                                                        defaults)
+
+    Identical to inspect.getargspec() in python2, but uses
+    inspect.getfullargspec() for python3 behind the scenes to avoid
+    DeprecationWarning.
+
+    >>> def f(a, b=2, *ar, **kw):
+    ...     pass
+
+    >>> _getargspec_py23(f)
+    ArgSpec(args=['a', 'b'], varargs='ar', keywords='kw', defaults=(2,))
+    """
+    if six.PY2:
+        return inspect.getargspec(func)
+
+    return inspect.ArgSpec(*inspect.getfullargspec(func)[:4])
+
+
 def get_func_args(func, stripself=False):
     """Return the argument name list of a callable"""
     if inspect.isfunction(func):
-        func_args, _, _, _ = inspect.getargspec(func)
+        func_args, _, _, _ = _getargspec_py23(func)
     elif inspect.isclass(func):
         return get_func_args(func.__init__, True)
     elif inspect.ismethod(func):
@@ -248,9 +267,9 @@ def get_spec(func):
     """
 
     if inspect.isfunction(func) or inspect.ismethod(func):
-        spec = inspect.getargspec(func)
+        spec = _getargspec_py23(func)
     elif hasattr(func, '__call__'):
-        spec = inspect.getargspec(func.__call__)
+        spec = _getargspec_py23(func.__call__)
     else:
         raise TypeError('%s is not callable' % type(func))