python: do not put generated files into the source tree
python: do not put generated files into the source tree

Untracked content shows up in git submodules, causing stuff to be marked
"dirty" by various git-level tools.

Tested by temporarily putting the resulting target within `ALL`. The
result is the same as without this change (and it remains unusable with
no generated index, at least when building out-of-tree, BTW).

CESNET/libnetconf2,CESNET/libnetconf2