chore: fix tests on py3.8

Tests were failing because the sanic dependency dropped support for
py3.8 in its current release. sanic is now pinned to the last compatible
version for py3.8 only.

Signed-off-by: Hal Blackburn <hwtb2@cam.ac.uk>

Author: h4l

requirements/test.txt

@@ -4,7 +4,12 @@ flake8-print
 pytest
 pytest-cov
 # web app tests
-sanic
+
+# sanic stopped supporting 3.8 in 24.12:
+# https://sanic.dev/en/release-notes/changelog.html#version-24120-
+sanic         ; python_version >= '3.9'
+sanic<24.12.0 ; python_version <  '3.9'
+
 sanic-testing
 aiohttp
 Pillow