The if in this change was not being reached in Python 2, because in
this version the repr includes a comma after the message. I have
chosen instead to do the same that was being done before commit
9f4cf295, but changing `message`
(which does not exist in Python 3) to `args[0]` which can be used in
both Python versions.

Newer versions of polib accept only unicode strings in the
`pofile` function (because they check if they start by the decoded
version of the BOM).
I changed the `data` that is passed to `pofile` to be a unicode
string in Python 2 too. This seems to work locally in my old
version of polib, so that at least the old behavior should be
kept the same.

- 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.