Search broken with read the docs theme in 3.4.1

[jenshnielsen]

To Reproduce
Build your docs using Sphinx 3.4.1 and sphinx-rtd-theme 0.5.0
Open the search page and try to search for something.
The spinner keeps spinning and no results are returned.
Opening the javascript console shows:

Uncaught ReferenceError: Stemmer is not defined
    query file:///C:/Users/Jens-work/source/repos/Qcodes/docs/_build/html/_static/searchtools.js:158
    setIndex file:///C:/Users/Jens-work/source/repos/Qcodes/docs/_build/html/_static/searchtools.js:98
    <anonymous> file:///C:/Users/Jens-work/source/repos/Qcodes/docs/_build/html/searchindex.js:1

Which is new compared to Sphinx 3.3.1

Building with Sphinx 3.3.1 or using the Alabaster theme and search works as expected.
I am not sure if this is a bug in Sphinx or the Read the docs theme.

Expected behavior

Search works as in 3.3.1

Your project
Qcodes build from this branch microsoft/Qcodes#2560

Screenshots

Environment info

• OS: Windows 10
• Python version: 3.7.9
• Sphinx version: 3.4.1
• Sphinx extensions: extensions = ['nbsphinx', 'sphinx.ext.autodoc', 'sphinx.ext.autosummary',
  'sphinx.ext.napoleon', 'sphinx-jsonschema', 'sphinx.ext.doctest',
  'sphinx.ext.intersphinx', 'sphinx.ext.todo',
  'sphinx.ext.coverage', 'sphinx.ext.mathjax',
  'sphinx.ext.viewcode', 'sphinx.ext.githubpages',
  'sphinx.ext.todo']
• Extra tools: Firefox/Edge sphinx_rtd_theme==0.5.0

[jenshnielsen]

This seems to already be fixed by readthedocs/sphinx_rtd_theme#1021 so there is probably not much that can be done from the sphinx side