4.2.1 release

[kghbln]

• Provide RELEASE-NOTES
• update COMPATIBILITY
• update INSTALL

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 40.12%. Comparing base (1c52d38) to head (739a800).

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #823 +/- ## ========================================= Coverage 40.12% 40.12% Complexity 2332 2332 ========================================= Files 78 78 Lines 8247 8247 ========================================= Hits 3309 3309 Misses 4938 4938 ```

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