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.

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.