Pin pytest-xdist to 1.23.3

1.23.4 broke pytest 3.4 compatibility, see:
pytest-dev/pytest#3724

Author: khink

setup.py

@@ -66,7 +66,7 @@
     'pytest>=3.6,<3.7',
     'pytest-cov==2.5.1',
     'pytest-django==3.1.2',
-    'pytest-xdist>=1.22<1.23',
+    'pytest-xdist==1.22.3',
     'tox>=3.0,<3.1',
 ]