Temporary limit pytest<5.4

Refs:
* Teemu/pytest-sugar#188
* pytest-dev/pytest#6931

setup.cfg

@@ -83,7 +83,11 @@ docs =
     sphinxcontrib-spelling>=4.3.0
 
 testing =
-    pytest>=4.6.6
+    # pytest >= 5.4.0 doesn't work with pytest-sugar <= 0.9.2
+    # once https://github.com/Teemu/pytest-sugar/pull/188 is
+    # merged, the upper bound can be removed.
+    # Ref: https://github.com/pytest-dev/pytest/issues/6931
+    pytest>=4.6.6,<5.4.0
     pytest-mock>=1.11.0
     pytest-sugar>=0.9.1
     pytest-testmon<1.0.0