Python's enums are not extensible, values can't be added and they can't
be subclassed. This is problematic if weboob devel adds new values,
because stable weboob will not be able to use them. They would need to
be replaced by a default value or None, which loses information.

Instead, implement custom enums that contain plain constants and support
adding values aftewards.

In Python 2, __repr__ and __str__ shouldn't return unicode.
This caused a problem for example when logging-debug a filter on Thumbnail.url,
as the logger couldn't encode the unicode repr.

flake8 --select W391,E302,E301,E304

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

I used autopep8 on some files and did carefully check the changes.
I ignored E501,E302,E231,E225,E222,E221,E241,E203 in my search, and at
least E501 on any autopep8 run.

Other style fixes not related to PEP8:
* Only use new-style classes. I don't think the usage of old-style
  classes was voluntary. Old-style classes are removed in Python 3.
* Convert an if/else to a one-liner in mediawiki, change docstring style
  change to a comment something that wasn't really appropriate for a
  docstring.
* Unneeded first if condition in meteofrance