Update Authlib and its usage

[lepture]

Authlib can parse id_token in an easy way. Just pass Google OpenID Connect discovery endpoint to oauth.register, it will handle everything for you.

Check this demo:

https://github.com/authlib/demo-oauth-client/blob/master/flask-google-login/app.py

[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

:memo: Please visit https://cla.developers.google.com/ to sign.

Once you've signed (or fixed any issues), please reply here with @googlebot I signed it! and we'll verify it.

---

What to do if you already signed the CLA

Individual signers

• It's possible we don't have your GitHub username or you're using a different email address on your commit. Check your existing CLA data and verify that your email is set on your git commits.

Corporate signers

• Your company has a Point of Contact who decides which employees are authorized to participate. Ask your POC to be added to the group of authorized contributors. If you don't know who your Point of Contact is, direct the Google project maintainer to go/cla#troubleshoot (Public version).
• The email used to register you as an authorized contributor must be the email used for the Git commit. Check your existing CLA data and verify that your email is set on your git commits.
• The email used to register you as an authorized contributor must also be attached to your GitHub account.

ℹ️ Googlers: Go here for more info.

[lepture]

I'm not going to sign this CLA. If you can't merge this PR without CLA, you can change the code yourself.

[evonide]

Hi Hsiaoming,

Thank you very much for the time to create a PR and the simplification :)! As I can't merge the PR, I've adjusted the code in https://github.com/google/vulncode-db/commit/bef3b0f24b7abe969e4266fe48e54e5a1a69756f as per your suggestion.