Update sphinx-copybutton requirement from ~=0.2.12 to ~=0.3.1 (#47)

Updates the requirements on [sphinx-copybutton](https://github.com/ExecutableBookProject/sphinx-copybutton) to permit the latest version.
- [Release notes](https://github.com/ExecutableBookProject/sphinx-copybutton/releases)
- [Changelog](https://github.com/executablebooks/sphinx-copybutton/blob/master/CHANGELOG.md)
- [Commits](executablebooks/sphinx-copybutton@v0.2.12...v0.3.1)

Signed-off-by: dependabot[bot] <support@github.com>

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

Author: dependabot[bot]

requirements-dev.txt

@@ -13,5 +13,5 @@ pytest ~=6.1.0
 ipython ~=7.18.1
 numpydoc ~=1.1.0
 sphinx ~=3.3.1
-sphinx-copybutton ~=0.2.12
+sphinx-copybutton ~=0.3.1
 sphinx_rtd_theme ~=0.5.0