_std: show caller source in deprecation message (#228)

Closes #227

Author: jayvdb

py/_std.py

@@ -15,8 +15,9 @@ def __init__(self):
         self.__dict__ = sys.modules
 
     def __getattr__(self, name):
-        warnings.warn("py.std is deprecated, plase import %s directly" % name,
-                      category=PyStdIsDeprecatedWarning)
+        warnings.warn("py.std is deprecated, please import %s directly" % name,
+                      category=PyStdIsDeprecatedWarning,
+                      stacklevel=2)
         try:
             m = __import__(name)
         except ImportError: