Add requirements test for box upgrade

tests/test_requirements.py

@@ -0,0 +1,23 @@
+"""Test availability of required packages."""
+
+import unittest
+from pathlib import Path
+
+import pkg_resources
+
+_REQUIREMENTS_PATH = Path(__file__).parent.with_name("requirements.txt")
+
+class TestRequirements(unittest.TestCase):
+    """Test availability of required packages."""
+
+    def test_requirements(self):
+        """Test that each required package is available."""
+        # Ref: https://stackoverflow.com/a/45474387/
+        requirements = pkg_resources.parse_requirements(_REQUIREMENTS_PATH.open())
+        for requirement in requirements:
+            requirement = str(requirement)
+            with self.subTest(requirement=requirement):
+                pkg_resources.require(requirement)
+
+test = TestRequirements()
+test.test_requirements()