pytest: use importlib mode by default
Otherwise the default (prepend) mode will add `<jax_src_dir>/tests` and
`<jax_src_dir>` to the start of `sys.path`. This is (has always been)
fragile, because `<jax_src_dir>` has `jax/` and `jaxlib/`
subdirectories, but it recently broke as `setuptools` 80 has a change in
behaviour for editable installations, with the result that if `jaxlib`
is installed editable then `import jaxlib` with `<jax_src_dir>` in
`sys.path` will try to import from `<jax_src_dir>/jaxlib` instead of the
editable install location, and fail.