Update for pytest 8.1
In our tests, we are using some patterns that are not supported by the
importlib
mode, like importing test modules from one another.
So switch back to the default prepend
mode, but add the
consider_namespace_packages
option that was introduced in pytest 8.1
so our swh
namespace package gets properly detected as such.
For reference, see: https://docs.pytest.org/en/latest/reference/reference.html#confval-consider_namespace_packages
(commit message from Lunar's fix in swh-alter).