Set minimum version of python-constraint (#574)

python-constraint 1.3.1 can't handle `AllDifferentConstraint()` correctly, this is a bug from upstream that must be fixed by a version upgrade

Co-authored-by: Umer Shahid <[email protected]>

Author: trdthg

requirements.txt

@@ -26,7 +26,7 @@ Pygments>=2.4.2
 pyparsing>=2.4.0
 pytablewriter
 pytest
-python-constraint
+python-constraint>=1.4.0
 python-dateutil>=2.8.0
 pytz>=2019.1
 pyyaml