Use a callable __tracebackhide__ for filtering

While this leads to slightly more complicated user code for the common
case (checking if the exception is of a given type) it's easier to
implement and more flexible.

doc/en/example/simple.rst

@@ -216,16 +216,18 @@ Let's run our little function::
     test_checkconfig.py:8: Failed
     1 failed in 0.12 seconds
 
-If you only want to hide certain exception classes, you can also set
-``__tracebackhide__`` to an exception type or a list of exception types::
+If you only want to hide certain exceptions, you can set ``__tracebackhide__``
+to a callable which gets the ``ExceptionInfo`` object. You can for example use
+this to make sure unexpected exception types aren't hidden::
 
+    import operator
     import pytest
 
     class ConfigException(Exception):
         pass
 
     def checkconfig(x):
-        __tracebackhide__ = ConfigException
+        __tracebackhide__ = operator.methodcaller('errisinstance', ConfigException)
         if not hasattr(x, "config"):
             raise ConfigException("not configured: %s" %(x,))