We should be able to configure a group calendar saying that the pattern
is "from 9:00 to 12:00, repeat every monday morning" with a group
calendar assignment saying "use this pattern from 01/01/2016 until
31/12/2016" and then create another group calendar assignment for 2017
without having to change the periodicity stop date on all presence
periods of the group calendar.

I think it should repeat from group calendar assignment's start date
until min(group calendar assignment's stop date, presence period's
periodicity stop date).

For example, Python Scripts were:
- dirty in the imported BT (causing all of them to be reinstalled on upgrade)
- compiled to return None

When a currency exchange line is (in)validated, we need to reindex the currency exchange
cells that acquire validation state from the currency exchange line.

This allows defining an exchange rate without end date

https://codemirror.net/demo/activeline.html

since be31425e

not only complete python language words but also all words from the
buffer.

/reviewed-on !122

Mark the test as expected failure for now.

This test is in expectedFailure state and is still not passing with this
change, but this test was showing wrong example that can cause
confusion.

Here are 2 examples of fixed cases:

- File object with data=None was exported with a second file containing the
  string 'None'

- Zope Pages Templates (and derived OOoTemplate) relied __setstate__ to fix
  the wrongly imported object.

This avoids computing a file extension that may not be used (if there's no data
to export), and fixes unreliable code duplication in _guessFilename().

Plus more implicit acquisition fixed.

We'll add another test checking that nothing is exported as a separate file.

Such images don't seem to exist.
On the other side, it may be useful to test raw data.

Note that the code to get data duplicates what's done later in export().
One should problably guess extension at the last moment.

Contrary to what the code looked like, it was always using /tmp.
Which could also cause conflict with other test suites running at the same time.

This was breaking the next test.