Update Coverage Workflow

- pin `pytest-xdist` to 1.34.0 as newer one requires pytest >= 6.0 and we are pinned at pytest==5.4.3 until caplog issue is resolved.

.github/workflows/coverage.yml

@@ -33,7 +33,7 @@ jobs:
       env:
         CFLAGS: '--coverage'
       run: |
-        pip install pytest==5.4.3 pytest-cov pytest-xdist
+        pip install pytest==5.4.3 pytest-cov pytest-xdist==1.34.0
         echo "::set-env name=PYTHONPATH::home/runner/work/psautohint/build/lib"
         python setup.py build --build-base build --build-platlib build/lib
         python setup.py install