Update sphinx-rtd-theme requirement from <4,>=2 to >=3.1.0,<4 (#2815)

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Jeffrey 'Alex' Clark <aclark@aclark.net>

Author: dependabot[bot]

requirements/docs.txt

@@ -1,5 +1,5 @@
 sphinx>=5.3,<9
-sphinx_rtd_theme>=2,<4
+sphinx_rtd_theme>=3.1.0,<4
 readthedocs-sphinx-search~=0.3
 sphinxcontrib-shellcheck>=1,<2
 sphinx-autobuild>=2020.9.1