🐛 fix for "expires_at" / "expires_in" param when "None" is specified

[stefanitsky]

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update
• Refactor
• Other, please describe:

Does this PR introduce a breaking change?

• Yes
• No

Description
Some providers provide endless access tokens, therefore, if you specify None for expires_at or expires_in - a token initialization error occurs.

For example, such services are GitHub and Gitlab. Moreover, in the case of the Gitlab, the token for updating is returned, but without expires_at (long-live bug and it won't be fixed, probably).

Links:

• Gitlab token: https://stackoverflow.com/questions/39264452/gitlab-oauth-access-token-validity
• GitHub token: https://stackoverflow.com/questions/26902600/whats-the-lifetime-of-github-oauth-api-access-token

• You consent that the copyright of your pull request source code belongs to Authlib's author.

[lepture]

Thanks

[fboudra]

Does it mean we can implement like Gitlab/Doorkeeper is doing and use authlib/OAuth 2 server to provide a custom access token expiration (endless)?