Use pytest < 5.3.5 to work pytest failure

pytest-dev/pytest#6925

requirements/test.txt

@@ -1,7 +1,7 @@
 attrs >= 19.1.0
 hypothesis >= 4.24, < 6
 Pillow >= 5.0.0
-pytest >= 4.4.0, < 6
+pytest >= 4.4.0, < 5.3.5
 pytest-xdist >= 1.28, < 2
 pytest-helpers-namespace >= 2019.1.8
 pytest-timeout >= 1.3.3