bitbake: lib/bs4: Fix imports from html5lib >= 0.9999999/1.0b8

As of html5lib 0.9999999/1.0b8 (released on July 14, 2016), some modules
have moved from _base to base. Handle this, while staying compatible
with earlier versions.

(Bitbake rev: edd90cd858da5da564e9849ebc0849a8a8173d4f)

Signed-off-by: Daniel Lublin <daniel@lublin.se>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>