* The brltty API was failing to work without polkit due to a bug.
Patched, and patch was accepted upstream.
Polkit is a recent optional dependency. Better to patch brltty's
bug than forcibly require it.
* Split out Python 3 bindings into python3-brlapi. This is the
package name used by Debian.
* Removed the a2 screen driver. It isn't needed, since
people using brltty with a GUI are going to be using Orca anyway.
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