Drop Python 2 support & enable GitHub Actions

[greyli]

• Drop Python 2 and 3.5 support.
• Enable CI for Python 3.6-3.10.
• Remove the copyright headers.
• Update project links.
• Update example applications to support Flask-SQLAlchemy 3.x.

fixes #60 fixes #61

[codecov-commenter]

Codecov Report

:exclamation: No coverage uploaded for pull request base (master@33439ff). Click here to learn what that means. Patch has no changes to coverable lines.

:mega: This organization is not using Codecov’s GitHub App Integration. We recommend you install it so Codecov can continue to function properly for your repositories. Learn more

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #62 +/- ## ========================================= Coverage ? 82.64% ========================================= Files ? 3 Lines ? 121 Branches ? 0 ========================================= Hits ? 100 Misses ? 21 Partials ? 0 ``` Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=helloflask). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=helloflask)

:umbrella: View full report at Codecov.
:loudspeaker: Do you have feedback about the report comment? Let us know in this issue.