Use extension helpers instead of manual setup

[WilliamJamieson]

This PR uses the extension-helpers build dependency to simplify the cython code build.

Checklist

• [ ] added entry in CHANGES.rst (either in Bug Fixes or Changes to API)
• [ ] updated relevant tests
• [ ] updated relevant documentation
• [ ] updated relevant milestone(s)
• [ ] added relevant label(s)

[codecov[bot]]

Codecov Report

Patch coverage has no change and project coverage change: -1.46% :warning:

Comparison is base (507cddd) 82.31% compared to head (ea2a1c7) 80.86%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #202 +/- ## ========================================== - Coverage 82.31% 80.86% -1.46% ========================================== Files 29 29 Lines 5554 5554 ========================================== - Hits 4572 4491 -81 - Misses 982 1063 +81 ``` | [Files Changed](https://app.codecov.io/gh/spacetelescope/stcal/pull/202?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=spacetelescope) | Coverage Δ | | |---|---|---| | [setup.py](https://app.codecov.io/gh/spacetelescope/stcal/pull/202?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=spacetelescope#diff-c2V0dXAucHk=) | `0.00% <0.00%> (ø)` | | ... and [3 files with indirect coverage changes](https://app.codecov.io/gh/spacetelescope/stcal/pull/202/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=spacetelescope)

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[pllim]

ImportError: cannot import name 'ols_cas22' from 'stcal.ramp_fitting'

Looks like a symptom of failed C-extension build. But I don't see any error in the log.