Drop no longer supported python versions

[heavenshell]

ref #636

ERROR: Could not find a version that satisfies the requirement pycodestyle>=2.9.0 (from autopep8==1.6.0) (from versions: 1.8.0.dev0, 2.0.0a1, 2.0.0, 2.1.0, 2.2.0, 2.3.0, 2.3.1, 2.4.0, 2.5.0, 2.6.0a1, 2.6.0, 2.7.0, 2.8.0)
ERROR: No matching distribution found for pycodestyle>=2.9.0 (from autopep8==1.6.0)

Unfortunally pycodestyle 2.9.0 does not support old version.
So I think it's time to drop old python version.

This PR remove old python version from GitHub action and tox

[hhatto]

ci is success in #640
merge this.

Thanks