Remove pytest-xdist from deps

Author: mryab

requirements-dev.txt

@@ -2,7 +2,6 @@ pytest==6.2.5  # see https://github.com/pytest-dev/pytest/issues/9621
 pytest-forked
 pytest-asyncio==0.16.0
 pytest-cov
-pytest-xdist>=3.0.2
 coverage==6.0.2  # see https://github.com/pytest-dev/pytest-cov/issues/520
 tqdm
 scikit-learn