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/__init__.py

@@ -84,7 +84,7 @@ def plain(*args):
     mainlogger.plain(''.join(args))
 
 def debug(lvl, *args):
-    if isinstance(lvl, basestring):
+    if isinstance(lvl, str):
         mainlogger.warning("Passed invalid debug level '%s' to bb.debug", lvl)
         args = (lvl,) + args
         lvl = 1