Pin to PyTest 8.0.0 (#5163)
Fix failures in nv-accelerate-v100 unit tests.
Fix running on this PR:
https://github.com/microsoft/DeepSpeed/actions/runs/7923894998/job/21634533848
Original failure
[here](https://github.com/microsoft/DeepSpeed/actions/runs/7961285826/job/21748664124?pr=5129#step:7:415):
```
from _pytest.doctest import (
E ImportError: cannot import name 'import_path' from '_pytest.doctest' (/tmp/actions-runner/_work/DeepSpeed/DeepSpeed/unit-test-venv/lib/python3.8/site-packages/_pytest/doctest.py)
```
