Update numpydoc requirement from ~=1.5 to ~=1.6 (#152)

Updates the requirements on [numpydoc](https://github.com/numpy/numpydoc) to permit the latest version.
- [Release notes](https://github.com/numpy/numpydoc/releases)
- [Changelog](https://github.com/numpy/numpydoc/blob/main/doc/release_notes.rst)
- [Commits](numpy/numpydoc@v1.5.0...v1.6.0)

---
updated-dependencies:
- dependency-name: numpydoc
  dependency-type: direct:production
...

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

Author: dependabot[bot]

requirements-docs.txt

@@ -10,7 +10,7 @@
 # It only exists to keep the list of dependencies in a separate file from
 # setup.py.
 ipython ~=7.34.0
-numpydoc ~=1.5
+numpydoc ~=1.6
 sphinx ~=6.2.1
 sphinx-copybutton ~=0.5.2
 sphinx_rtd_theme ~=1.3.0