mail body is str

 - Products.PythonScripts now has a __loader__
 - do not use < > as filename for component on py3, this makes linecache
 work out of the box. On py2 I think it was causing errors trying to
 actually open the file, but this does not seem needed on py3 and
 simplifies everything

new test with ERP5-style workflow to cover python3 migration

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.