Fix 3.8 errors

[tylerwince]

Fixes #494

[tylerwince]

@ericwb This fixes 3.8 so people can start using this on the beta again

[tylerwince]

Just want to ping this to the top of your inbox @ericwb

[ericwb]

Very nice! LGTM

[adideutsch]

Hi @ericwb @tylerwince ,

Not sure where this conversion exactly belongs. I've also opened an issue here.

Some background -
I am using bandit with Python 3.8.6 for a few months now. Running bandit today using the new version 1.6.3 (thanks!), I found out about new issues (B104, B608) that were not visible before (job completed successfully).
Initially, I suspected that the CLI tool on version 1.6.2 didn't parse some args properly until the update today (I noticed several new lines on today's run: "Using command line arg for ..." which were missing before). However, I am not really sure that's really the reason - other issues did appear in the past using the exact same test job.

Any idea why these issues were identified today all of a sudden? Being unable to pinpoint why this happened, I'm worried there might be other undetected issues.
If this has happened because of Python 3.8 support issues, how can users be aware that bandit is not fully supported by their current python version?
Implementation suggestion for 2 - modify 'python_requires)' on setup.py to have an upper limit as well, thus making installations fail on supported versions.

Thanks,
Adi