But it's really not enough. A deep rework of documentation on this
module is mandatory. First, explain each objects:

 * Subscription at top level
 * Each subscription can have a list of Document/Detail/Bill
 * Explain differences between get_details and iter_history
 * Explain what is a balance (it's a "Detail" object! We are crazy
   sometimes)
 * Clean all references of phones/lines. Since the capability was made
   for something like "CapBillTelefon" at begin, it's not clean
   everywhere

Some functions changed return type from python2 to python3

TODO: not everything is tested, a full review is needed + run is needed

All iter_bills_history are renamed in modules by patch
28f94119
But since boobill call iter_bills_history (and probably other scripts),
they are now broken, all modules are raising a NotImplementedError

flake8 --select W391,E302,E301,E304

autopep8 can't fix W391 even though it claims it can.
Fixed using a simple custom script.

Closes #1202

closes #1203

Remove useless trailing \
Remove trailing spaces
Add missing empty lines

autopep8 -ir -j2 --select=E301,E302,E502,W291,W293,W391 .

Diff quickly checked.

Useful for pre-paid subscription

Modules can raise this exception when they want to print errors to user

And add restrict_level helper method.