build(deps): bump pysiaf from 0.20.0 to 0.21.0 (#763)

Bumps [pysiaf](https://github.com/spacetelescope/pysiaf) from 0.20.0 to 0.21.0.
- [Release notes](https://github.com/spacetelescope/pysiaf/releases)
- [Changelog](https://github.com/spacetelescope/pysiaf/blob/main/CHANGES.rst)
- [Commits](spacetelescope/pysiaf@v0.20.0...v0.21.0)

---
updated-dependencies:
- dependency-name: pysiaf
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

requirements.txt

@@ -4,7 +4,7 @@ matplotlib==3.8.1
 numpy==1.26.1
 photutils==1.9.0
 poppy>=1.0.0
-pysiaf==0.20.0
+pysiaf==0.21.0
 scipy==1.11.3
 synphot==1.2.1
 astroquery==0.4.6