This includes heuristics to guess the str or bytes from business template
XML: oids are bytes and also some strings that do not decode to UTF-8,
so that we can install python2 business templates on py3.

When exporting business templates, we need to build a list of referenced
persistent objects to export them separately in the XML, this is is done
using noload, in a way which does not support pickle protocol 1 on py3
(the persistent ids are None and the assertion in
https://github.com/zopefoundation/ZODB/blob/d698507bb89eeb38c6e655199bc9f54c909dbf4d/src/ZODB/serialize.py#L669
fails), so we need to use pickle protocol 3 on py3.

With these changes, on py2 we are still using the same protocol 1 and
the output is stable, but on py3 we are using protocol 3 and the output
is different. Business template exported on py

This also adds some test coverage of XMLExportImport, while doing this
it was found that the ad-hoc handling of boolean in protocol 1 was not
implemented correctly and they were serialized as integers (0 for False
and 1 for True), this is also fixed.

mail body is str

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.