Warn about deprecations only if token decoding succeeds

[anakinj]

Description

This should fix the spammy deprecation warnings reported in #595

Checklist

Before the PR can be merged be sure the following are checked:

• There are tests for the fix or feature added/changed
• A description of the changes and a reference to the PR has been added to CHANGELOG.md. More details in the CONTRIBUTING.md

[anakinj]

@jeremyevans If you have some spare time could you check out this attempt to warn about the upcoming stricter base64 decoding only if the token validation is a success.

[jeremyevans]

I didn't test this code, but reading it, it looks like if you rescue the exception, the warning won't be emitted then, but it will be emitted later, after the next attempt that does not raise in the same thread.

I'll submit a pull request for an alternative approach and get your thoughts about it.

[anakinj]

You were right, the warnings leaked over to the next call of decode. Did a little adjustment to prevent that.

[jeremyevans]

You were right, the warnings leaked over to the next call of decode. Did a little adjustment to prevent that.

My approach in #601 is simpler, does not mess with thread-local state, and uses uplevel to make it easier to find the underlying cause of the problem. It does require base64 decoding segments twice, but as most JWTs are small, I don't expect that should be a significant performance issue.

However, either #601 or this PR will fix the problem, so I'm fine with either.

[anakinj]

@jeremyevans Thanks for your interest and insight on this one. I decided to go with this more complex and hacky version. Just to avoid doing something for every token decode that goes via that method.