Fix cache bug if index version is too new

[hunse]

Motivation and context:

If the cache version is too new, we should just ignore the cache. Currently, there's an error because it tries to use the index when it's None.

How has this been tested?

I added a test.

How long should this take to review?

• Quick (less than 40 lines changed or changes are straightforward)

Types of changes:

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• I have read the CONTRIBUTING.rst document.
• I have updated the documentation accordingly.
• I have included a changelog entry.
• I have added tests to cover my changes.
• I have run the test suite locally and all tests passed.

[hunse]

This was caused by 4caba86, because we changed a number of except Exception statements to more specific exceptions, but obviously missed catching the exception that causes this problem. We might want to consider switching a few of those back and just disabling the pylint warning (in cache.py specifically).

[jgosmann]

I think this will print a "Decoder cache could not acquire lock and was deactivated." warning even though it might be a version mismatch. While this broke prior to this PR, it might make sense to fix that as part of this PR?

[jgosmann]

Could use (CacheIndex.VERSION + 1, pickle.HIGHEST_PROTOCOL + 1) to make it perfectly clear that these number are supposed to be higher than the current version numbers and avoid the "magic" numbers (but this is not a blocker to me).