bitbake: bitbake: Convert to python 3

Various misc changes to convert bitbake to python3 which don't warrant
separation into separate commits.

(Bitbake rev: d0f904d407f57998419bd9c305ce53e5eaa36b24)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

bitbake/lib/bb/command.py

@@ -110,7 +110,7 @@ class Command:
             return False
         except SystemExit as exc:
             arg = exc.args[0]
-            if isinstance(arg, basestring):
+            if isinstance(arg, str):
                 self.finishAsyncCommand(arg)
             else:
                 self.finishAsyncCommand("Exited with %s" % arg)