Pledge to drop 2.7 support

[tylerwince]

The https://python3statement.org/ pledge to drop support for Python2.7 when it hits end of life on January 1, 2020 should probably be taken up by this project. We could also add a test to check for any Py2 specific stuff and create a new error saying it is not longer receiving security updates and should be avoided.

Happy to make the PR on the py3statement project on behalf of bandit if this is something the maintainers are interested in.

[ericwb]

I agree, it would be good to indicate future dropping of Py27. However for https://python3statement.org/, I would expect them to only take PRs on such statements from maintainers of Bandit.

In any case, we should make changes in Bandit to note the upcoming removal. README and docs definitely. Maybe even a runtime message.

[tylerwince]

Probably the best thing would be to just note the 1.x series will be the last to support Py2.7 and make the changes in 2.0 to be 3 only

[ericwb]

1.6.3 is out on PyPI which includes drop of Py2.7 Thanks!