iter_documents() now returns documents sorted by date, from newest to oldest

Following suggestion by @ntome in !446.
Additional helper functions (sorted_documents, merge_iterators) have been
created in (new) weboob/tools/capabilities/bill/documents.py based on
their counterparts in weboob/tools/capabilities/bank/transactions.py
(generalisation should be possible at some point)
7 jobs for free-add-contracts in 16 minutes and 33 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Build
passed #28691
build:3

00:01:02

 
  Test
passed #28694
load-modules:3

00:02:22

passed #28692
pyflakes:3

00:02:25

passed #28693
pyflakes-strict:3

00:02:29

passed #28695
unittests:3

00:02:02

failed #28696
allowed to fail
unittests-modules:3

00:11:28

 
  Deploy
passed #28697
doc:3

00:01:50

 
Name Stage Failure
failed
unittests-modules:3 Test
-----------------------------------------------------------------------
TOTAL 70512 41149 42%
----------------------------------------------------------------------
Ran 370 tests in 513.864s

FAILED (SKIP=112, errors=150, failures=34)
=== Total coverage ===
MODULES COVERAGE: 42%
ERROR: Job failed: exit code 1