Update sphinx-rtd-theme requirement from ~=1.1.1 to ~=1.2.0 (#138)

Updates the requirements on [sphinx-rtd-theme](https://github.com/readthedocs/sphinx_rtd_theme) to permit the latest version.
- [Release notes](https://github.com/readthedocs/sphinx_rtd_theme/releases)
- [Changelog](https://github.com/readthedocs/sphinx_rtd_theme/blob/master/docs/changelog.rst)
- [Commits](readthedocs/sphinx_rtd_theme@1.1.1...1.2.0)

---
updated-dependencies:
- dependency-name: sphinx-rtd-theme
  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

@@ -13,4 +13,4 @@ ipython ~=7.34.0
 numpydoc ~=1.5
 sphinx ~=5.3.0
 sphinx-copybutton ~=0.5.2
-sphinx_rtd_theme ~=1.1.1
+sphinx_rtd_theme ~=1.2.0