sphinx_rtd_theme extension does not declare if it is safe for parallel reading

[Holzhaus]

Problem

The extension causes a warning when sphinx-build is used with -j $(nproc).

Error Logs/Results

WARNING: the sphinx_rtd_theme extension does not declare if it is safe for parallel reading, assuming it isn't - please ask the extension author to check and make it explicit
WARNING: doing serial read

Expected Results

Please fix the warning. This could also improve build times.

Environment Info

• Python Version: 3.7
• Sphinx Version: 3.0.4
• RTD Theme Version: 0.4.3

[p12tic]

This has already been fixed in #874.

[Holzhaus]

On second thought, let's leave this open until 0.5.0 has been released.

[agjohnson]

0.5.0 is now out and it should have these declarations on the theme extension