Fix import in nbconvert.py
Fix problem with missing .index_generator

Remove open-coded implementation of itertools generator.
Do no use bare except:

except: catches BaseException, which includes KeyboardInterrupt,
which means that ^C doesn't work.
Decode user input into unicode

Author name: Gąbal
...

Traceback (most recent call last):
...
  File "./nbconvert/converters/../templates/skeleton/tex/null.tplx", line 25, in top-level template code
    ((*- block header -*))
  File "./nbconvert/converters/../templates/tex/latex_sphinx_base.tplx", line 141, in block "header"
    \renewcommand{\releasename}{((( nb.metadata._draft.release | escape_tex )))}
UnicodeDecodeError: 'ascii' codec can't decode byte 0xc4 in position 1: ordinal not in range(128)