fixing missing exception when signature expired and change submodule from git to https git

[vicding-mi]

This PR fixes 2 issues I found when using flask-oidc pack.

1. When the token expires, it raise an exception of type SignatureExpired, this exception class is in the package but not imported.
2. when installing flask-oidc from GitHub using pip -r requirements.txt, it gives an error while cloning the submodule. Changed the protocol of submodule from git to https solves this issue.

The second issue only occurs when installing directly from GitHub. And users need the master branch on GitHub as it fixes issues which block usual auth action and always return 401 Not Authorised