Fix the edge case resulting in incorrect contains resolution

- Pairwise constraint evaluation misses the case when filtered
 constraints contains only one item
- Fixes #137

Signed-off-by: Keshav Priyadarshi <git@keshav.space>

Author: keshav-space

src/univers/version_constraint.py

@@ -506,6 +506,10 @@ def contains_version(version, constraints):
     if not constraints:
         return False
 
+    # If we end up with constraints list contains only one item.
+    if len(constraints) == 1:
+        return version in constraints[0]
+
     # Iterate over the current and next contiguous constraints pairs (aka. pairwise)
     # in the second list.
     # For each current and next constraint: