Manually set pytest version until error is resolved

As part of the upgrade from pytest 8.0.0 -> 8.1.0, a deprecated
function is used, which only emitted a warning before but since
8.1.0 it raises an error upon usage. So, if the usage of this
function is changed in a future pytest release, the hardcoded
version can be removed again ([ref]
(pytest-dev/pytest#11779)).

Author: 8R0WNI3

.ci/test

@@ -2,6 +2,8 @@
 
 set -e
 
+pip3 install pytest==8.0.0 # pytest 8.1.0 uses a deprecated function which raises an error
+
 if ! which pytest &>/dev/null; then
   echo "pytest is required (install with pip(3) install pytest)"
   exit 1