Add an ignore to flake8 in tox.ini.

Fixes #431 (closed) flake8 3.5.0 has a new E722 for bare except: clauses. Bare except: seems appropriate for those 5 places they occur. Ignoring them with # noqa: E722 causes diffcov to fail on 3 of them, and covering these with new tests is problematic.

This MR adds an ignore directive to tox.ini to ignore E722 and those other flake8 errors we encounter that are ignored by default.