Drop py36 support

[jpdakran]

• Please check if the PR fulfills these requirements

• [ ] Tests for the changes have been added

• [x] Docs have been added / updated

• [x] All CI checks are green

• What kind of change does this PR introduce?

• Feature

• What is the current behavior?

• Currently detect-secrets supports py36-39.

• What is the new behavior (if this is a feature change)?

• Drop py36 support as it has reached EOL on December 23, 2021.

• Does this PR introduce a breaking change?

• Yes. Upgrade python versions.

• Other information:

[lorenzodb1]

Should we add support for python 3.10 too?

[lorenzodb1]

Also, fyi py37 EOL is set to be in three months (source), so let's keep that in mind too.

[jpdakran]

@lorenzodb1 Yea good idea about py310. I just did a quick smoke test and there may be some dependency modifications. I'll make a note about it for a future issue - just to separate out the work.