add pep8 options to tox.ini

Add the pep8 section to tox.ini and specify ignoring E113.
Many E113 errors appeared on comments. It seems a bug of pep8, so it can be ignored.

Signed-off-by: Yoshihiro Kaneko <ykaneko0929@gmail.com>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>