tests were failing because of pep8 issues

For now, the way to select a gateway is to add a section in
bitmaskd.cfg:

  [vpn_prefs]
  locations = ["frankfurt", "seattle__wa"]
  countries = ["DE", "US"]

Note that the location indication has priority over country code.

This will be exposed by the UI in release 0.11

- Resolves: #8855

This is a first approach to automatic gateways selection.
More things are missing:

- allow manual selection, by location or country code.
- take the hemisphere into account.
- expose the selected gw to the api/cli

but overall seems this is a good approach to make 0.10 release usable in
terms of vpn.

- Resolves: #8804

When setting the listeners in the IMAP Folder, we avoid setting more
than one listener for the same imap mailbox (because in some situations
we were registering way too many listeners).

this was making the pixelated inbox registering the notification and
therefore the imap mailbox not being registered.

this MR also refactors the way pixelated is initialized, so that it
avoid creating a second Account instance. In this way, we make sure that
the pixelated mua and the imap server share the same collections for a
given mailbox, and therefore any of the two is able to get a
notification whenever the other adds a message to the mailbox.

- Resolves: #8846, #8798