build(deps): bump pytest-doctestplus in /requirements

Bumps [pytest-doctestplus](https://github.com/scientific-python/pytest-doctestplus) from 1.5.0 to 1.6.0.
- [Release notes](https://github.com/scientific-python/pytest-doctestplus/releases)
- [Changelog](https://github.com/scientific-python/pytest-doctestplus/blob/main/CHANGES.rst)
- [Commits](scientific-python/pytest-doctestplus@v1.5.0...v1.6.0)

---
updated-dependencies:
- dependency-name: pytest-doctestplus
  dependency-version: 1.6.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

requirements/doctests.txt

@@ -1,2 +1,2 @@
 pytest ==8.4.2
-pytest-doctestplus ==1.5.0
+pytest-doctestplus ==1.6.0