Update sphinx-rtd-theme requirement from <1.4,>=1.0 to >=1.0,<2.1

Updates the requirements on [sphinx-rtd-theme](https://github.com/readthedocs/sphinx_rtd_theme) to permit the latest version.
- [Changelog](https://github.com/readthedocs/sphinx_rtd_theme/blob/master/docs/changelog.rst)
- [Commits](readthedocs/sphinx_rtd_theme@1.0.0...2.0.0)

---
updated-dependencies:
- dependency-name: sphinx-rtd-theme
  dependency-type: direct:development
...

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

setup.cfg

@@ -45,7 +45,7 @@ packages =
 
 [options.extras_require]
 docs =
-    sphinx_rtd_theme>=1.0,<1.4
+    sphinx_rtd_theme>=1.0,<2.1
     sphinx-issues~=3.0.0
 dev = 
     tox