See merge request nexedi/erp5!1922

See merge request nexedi/erp5!1929

See merge request nexedi/erp5!1926

See merge request nexedi/erp5!1927

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 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.

- 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.

Fixes an assert due to a unicode/bytes mismatch.

Drop old test adjustments, we only test DateTime >= 3 nowadays.

Make the condition for test_pickle_old_with_micros_as_float based on
python version for simplicity.

this is removed from python3, it was not so useful here either