Explicitly request pytest-xdist<2 when using pytest 5.3

Unfortunately this is a problem with pip's solver

Author: nicoddemus

tox.ini

@@ -4,9 +4,10 @@ envlist = py{27,35,36,37,38,38}-pytestlatest,py38-pytest53
 [testenv]
 deps=
     pytestlatest: pytest
-    pytest53: pytest ~=5.3
+    pytestlatest: pytest-xdist
+    pytest53: pytest~=5.3
+    pytest53: pytest-xdist<2
     pytest-mock
-    pytest-xdist
     coverage
 commands=
     coverage run --source=pytest_cpp -m pytest tests