Use codecov-action v4

[skirpichev]

It seems, https://app.codecov.io/gh/aleaxit/gmpy - not updating. And coverage for new pull requests is missing too: https://app.codecov.io/gh/aleaxit/gmpy/pulls?prStates%5B0%5D=OPEN

This update codecov-action version and add fail_ci_if_error option to prevent CI failures on the master. In this form, submitting data works in mpmath's prs.

If someone menages to add CODECOV_TOKEN (as it's explained in https://github.com/codecov/codecov-action/?tab=readme-ov-file#usage)

• then we could remove fail_ci_if_error option (defaults to true).

[codecov-commenter]

Codecov Report

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

Project coverage is 85.50%. Comparing base (224745b) to head (374b37f). Report is 24 commits behind head on master.

:exclamation: Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #487 +/- ## ========================================== - Coverage 85.52% 85.50% -0.03% ========================================== Files 50 50 Lines 11656 11645 -11 Branches 2202 2201 -1 ========================================== - Hits 9969 9957 -12 - Misses 1687 1688 +1 ```

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[casevh]

Thanks.