ENH: Schedule int64 warning to convert to error at 5.0

[effigies]

Follow-up to #1082, which will need to transition from a warning to an error on the release of 5.0 (which I'm aiming for next week).

Created a new function in nibabel.deprecated to turn what we need to do here into a template that can be used elsewhere.

[codecov]

Codecov Report

Base: 92.37% // Head: 92.37% // No change to project coverage 👍

Coverage data is based on head (e662e55) compared to base (e662e55).
Patch has no changes to coverable lines.

❗ Current head e662e55 differs from pull request most recent head 0aeecbc. Consider uploading reports for the commit 0aeecbc to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1173   +/-   ##
=======================================
  Coverage   92.37%   92.37%           
=======================================
  Files          97       97           
  Lines       12243    12243           
  Branches     2524     2524           
=======================================
  Hits        11310    11310           
  Misses        613      613           
  Partials      320      320           

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

☔ View full report at Codecov.
📢 Do you have feedback about the report comment? Let us know in this issue.

[effigies]

@matthew-brett Any opinions?

[effigies]

This just needs to happen. I'm going to merge...