Workaround for RTD not showing colons for function args

[ianthomas23]

Fixes #790 which is that Read the Docs does not display colons between argument names and types.

There is a discussion about this on the sphinx_rtd_theme github readthedocs/sphinx_rtd_theme#766. The workaround here is the one recommended in that issue.

This PR is made on top of #1149 so that RTD will successfully build the docs so the workaround can be checked. Ideally #1149 should be merged before this.

Longer term it might be worth us considering a switch from the RTD theme to one that is more actively maintained. Options that work well for relatively small documentation projects are furo (https://github.com/pradyunsg/furo) and sphinx-book-theme (https://github.com/executablebooks/sphinx-book-theme).

[ianthomas23]

Here is some output from the RTD build showing that the fix works.

[martindurant]

Perfect, thank you.