tools: use functools.wraps in deprecated

The way it worked before, Sphinx would output `**kwargs` as the function
parameters for methods decorated with `deprecated`, instead of the real
signature. Not surprising, I guess, given what the code was doing.

As for why we weren't already using functools.wraps in the decorator
itself, it's been around since the root commit of this repo, ten years
ago, back when this project was still named "Jenni". The functools
module was added in Python 2.5, but maybe this decorator was originally
added before that version came out? We might never know, but let's use
the modern way from now on!

sopel/tools/__init__.py

@@ -14,11 +14,12 @@
 
 from __future__ import unicode_literals, absolute_import, print_function, division
 
-import sys
+import codecs
+import functools
 import os
 import re
+import sys
 import threading
-import codecs
 import traceback
 from collections import defaultdict
 
@@ -232,14 +233,13 @@ def deprecated(old):
     Any time a decorated function is used, a deprecation warning will be printed
     to the console/log-file.
     """
+    @functools.wraps(old)
     def new(*args, **kwargs):
         print('Function %s is deprecated.' % old.__name__, file=sys.stderr)
         trace = traceback.extract_stack()
         for line in traceback.format_list(trace[:-1]):
             stderr(line[:-1])
         return old(*args, **kwargs)
-    new.__doc__ = old.__doc__
-    new.__name__ = old.__name__
     return new