upgrade xdist for sys.path fix

Author: graingert

tox.ini

@@ -30,7 +30,7 @@ deps =
     pytest43: pytest~=4.3.0
 
     coverage: pytest-cov
-    xdist: pytest-xdist
+    xdist: pytest-xdist>=2.3.0
     -r{toxinidir}/requirements-testing.txt
     anyio[trio]
 commands = {env:_PYTEST_CMD:pytest} {env:_PYTEST_MORE_ARGS:} {posargs:-vvl}