Resolve "improve coding style"

flake8 | wc -l
15

see setug.cfg for ignored errors.

This made in relief other issues (not fixed in this MR):

flake8 | grep F821
./bt/multifile.py:74:19: F821 undefined name 'validationError'
./bt/sheet.py:46:11: F821 undefined name 'ooolib'
./bt/views.py:308:20: F821 undefined name 'HttpResponseNotFound'

Closes #65 (closed)