IkiWiki's highlight plugin requires this.

Note that the current highlight version (3.57) is not compatible with
ikiwiki version in nixpkgs. This will be fixed in the next commit.

This fixes (among others) incompatibility with highlight from nixpkgs.
See http://source.ikiwiki.branchable.com/?p=source.git;a=commit;h=4d06df9583e6c4145f8c6fc2fd51d7894c0b85ce

The line-continuation backslash should not be a part of optional
string because it can lead to errors like this:

    /nix/store/333six1faw9bhccsx9qw5718k6b1wiq2-stdenv-linux/setup: line 90: --prefix: command not found
    builder for '/nix/store/lb07r900fpg7r1555zpf0j8n1n7g3wlx-ikiwiki-3.20200202.3.drv' failed with exit code 127

Without this, the rst plugin complains as follows:

    cannot import docutils.core

Also, python3 is needed for the rst plugin to work.