Bump sphinx-rtd-theme from 2.0.0 to 3.0.0 in /doc

Bumps [sphinx-rtd-theme](https://github.com/readthedocs/sphinx_rtd_theme) from 2.0.0 to 3.0.0.
- [Changelog](https://github.com/readthedocs/sphinx_rtd_theme/blob/master/docs/changelog.rst)
- [Commits](https://github.com/readthedocs/sphinx_rtd_theme/compare/2.0.0...3.0.0)

---
updated-dependencies:
- dependency-name: sphinx-rtd-theme
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

doc/requirements.txt

@@ -2,7 +2,7 @@ pyusb
 Pillow>=2.0
 qrcode>=4.0
 pyserial
-sphinx-rtd-theme==2.0.0
+sphinx-rtd-theme==3.0.0
 setuptools
 setuptools-scm
 docutils>=0.12