www/buku: add USE_PYTHON=noflavors

[00:01:53] Warning: (www/buku@py35): Error: Duplicated origin for buku-4.0: www/buku@py35 AND www/buku. Rerun with -v to see which ports are depending on these.
[00:01:53] Warning: (www/buku@py37): Error: Duplicated origin for buku-4.0: www/buku@py37 AND www/buku. Rerun with -v to see which ports are depending on these.

Approved by: portmgr (antoine)