The `details` part of the output of functional test cases was
being wrongly converted to an unicode string in Python 2.
This caused problems when the output was sent to a file
object not supporting non-ASCII code points.
This commit fixes that behaviour.

Some tests were added to check warnings, which were not working in
Python 2 because the warning message was shown just once.
This is fixed by adding a `warnings.simplefilter("always")` inside
the `catch_warnings` context manager, as recommended in
https://docs.python.org/3/library/warnings.html#testing-warnings .
Note that the previous filter is restored on leaving the context
manager.

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.

An unicode encoding error was being raised in
test_AccountingTransaction_getListBoxColumnList_item_column in Python 2,
when the `html` variable was being passed to the `StringIO`
constructor. The error was only reproducible when running the tests in
the CLI.

The problem was that we were using cStringIO module in Python 2, and the
string فارسی (farsi) appears in the HTML (probably inside a language
picker). cStringIO cannot parse unicode characters that are non-ASCII.
The solution is to use "six.StringIO", which uses the (slower,
but unicode-compatible) StringIO module in Python 2.

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

__getitem__ was missing causing unsubscriptable-object on BTrees