- The warnings are compared in a set instead of a list, as we do
not care of how many times is the warning raised.
- The warning filter is added inside the context manager, as
recommended in
https://docs.python.org/2.7/library/warnings.html#testing-warnings .
- We change the `clear` method of list, using `del` instead, as the
`clear` method was added on Python 3.3.

Instead of using encode/decode is it better to use str2bytes and
bytes2str, so that the types in Python 2 are unchanged. More important,
this prevents the double encoding of bytes, which causes problems
when they have non-ASCII data encoded.

Errors were being raised for returning `None` in Python 2
(due to a missing else branch) and for re-encoding bytes
in Python 2.

I changed the code to use str2bytes in the cases where bytes
are expected.

( I'm wondering if we need this patch at all on py3 )

__getitem__ was missing causing unsubscriptable-object on BTrees