use pytest 5.3.3 explicitely for xdist tests

Author: SalmonMode

tox.ini

@@ -30,6 +30,7 @@ deps =
     pytest41: pytest>=4.1,<4.2
     pytest41: attrs==17.4.0
     xdist: pytest-xdist>=1.15
+    xdist: pytest==5.3.3
 
 setenv =
     PYTHONPATH = {toxinidir}:{env:PYTHONPATH:}