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.