weboob.capabilities: replace stdlib enum with custom enums

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.
10 jobs for enumrevert in 0 seconds (queued for 1614 minutes and 33 seconds)
Status Job ID Name Coverage
  Build
failed #12060
build:2.7

failed #12061
build:3

 
  Test
skipped #12063
allowed to fail
lint:2.7
skipped #12066
allowed to fail
lint:3
skipped #12062
pyflakes:2.7
skipped #12065
pyflakes:3
skipped #12064
unittests:2.7
skipped #12067
unittests:3
 
  Deploy
skipped #12068
doc:2.7
skipped #12069
doc:3
 
Name Stage Failure
failed
build:2.7 Build There is an unknown failure, please try again
No job trace
failed
build:3 Build There is an unknown failure, please try again
No job trace