Add pytest-cases development dependency

The addition of pytest-cases allows for greater control over how to
implement test cases that uses different file contents for each test case.

Author: connesy

dev-requirements.txt

@@ -2,5 +2,6 @@ black ~= 23.1
 isort ~= 5.12
 mypy ~= 1.0
 pytest ~= 7.4
+pytest-cases ~= 3.6
 pytest-order ~= 1.1
 pytest-xdist ~= 3.3