Fix wrong traceback in deprecation warnings on Python 3.5.0

Python 3.5.0 has a bug in the `traceback.extract_stack` method, which
returns an extra internal stack frame. This causes some trouble in the
nice deprecation warnings message, because the hint about where to
change things points to a different file (and line number).

This commit adds a special workaround activated only on Python 3.5.0.

Upstream issue: http://bugs.python.org/issue25108

Author: Pietro Albini

botogram/utils.py

@@ -73,6 +73,12 @@ def get(k):
 
 def warn(stack_pos, before_message, after_message=None):
     """Issue a warning caused by user code"""
+    # This is a workaround for http://bugs.python.org/issue25108
+    # In Python 3.5.0, traceback.extract_stack returns an additional internal
+    # stack frame, which causes a lot of trouble around there.
+    if sys.version_info[:3] == (3, 5, 0):
+        stack_pos -= 1
+
     frame = traceback.extract_stack()[stack_pos-1]
     at_message = "At: %s (line %s)" % (frame[0], frame[1])