Phylo.draw_ascii: use floor division (Py3 compatibility)

See Bug #3435
Using // for explicit integer division, don't need import
SeqIO: Better cope with wrong LOCUS line spacing in GenBank

Signed-off-by: Kai Blin <kai.blin@biotech.uni-tuebingen.de>
Fix minor typo in exception
Thank Kai for GenBank parsing work
For Python 3, recommend Python 3.3.2 or later
Run part of test_Phylo_depend.py if NetworkX missing