Bug 1823701 - Add 'tomli' to mach wpt requirements, r=jgraham

Since we added a root `pyproject.toml` file, it triggered a code path in pytest
which tries to open the file to read configuration with `tomli`. For whatever
reason, this isn't vendored for wpt and we therefore get import errors.

Differential Revision: https://phabricator.services.mozilla.com/D173166

python/sites/wpt.txt

@@ -1,3 +1,6 @@
 pypi:tox==3.12.1
 pypi:ujson==4.0.2
 pypi:aioquic==0.9.19
+# Bug 1823701 - tomli is a pytest dependency that isn't pulled into WPT's
+# vendor dir for some reason
+pypi:tomli==2.0.1