Log Message:
v4.1.0

Note: oauth2client is now deprecated. No more features will be added to the
libraries and the core team is turning down support. We recommend you use
google-auth and oauthlib.

New features:
* Allow customizing the GCE metadata service address via an env var.
* Store original encoded and signed identity JWT in OAuth2Credentials.
* Use jsonpickle in django contrib, if available.

Bug fixes:
* Typo fixes.
* Remove b64 padding from PKCE values, per RFC7636.
* Include LICENSE in Manifest.in.
* Fix tests and CI.
* Escape callback error code in flask_util.