Improve stoqlib.domain.inventory a little

* Make the docstrings more readable and more compatible with PEP 257.
* Remove 'get_open_branches' method. It didn't have any callsite and it simple
  doesn't make sense from the Inventory point of view (maybe if the name was
  different)
* Remove a not used param from 'close' method. Every callsite was calling
  close() directly (except for a test, but it can modify close_date directly),
  so make the api simpler and remove that keyword

Change-Id: Ifa606e27da24051c01f708ec0ed9a910cfafa05a
Merge "Improve stoqlib.domain.inventory a little"