new test with ERP5-style workflow to cover python3 migration

The e-mail messages are normalized in a way compatible with both
Python 2 and Python 3.

In Python 2, the previous version added a `unixfrom`, which contains
the date, making the tests fail when there is a change of one
second in the clock between generating the first and the second
message.
Moreover, the second message contained a previous `unixfrom`.
This has been solved by removing `unixfrom` in the normalized string.

In Python 3, messages had an inconsistent folding, due to
https://github.com/python/cpython/issues/72649 .
This has been solved by manual unfolding (using a regex substitution)
and by outputing normalized strings without folding (setting
`maxheaderlen=0`, which means no maximum line lenght).

With these changes, the normalized string can be compared in both
versions without problems.

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.

Fixes an assert due to a unicode/bytes mismatch.

- 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.

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

redefine-in-handler does not exist on pylint 3 and was no longer emitted here
on python2.