Fix np.is_in typo in cps.marriage_tax_reforms (#257) (#1637)

`np.is_in` is not a numpy function — the correct name is `np.isin`.
Both call sites sit inside `if child_education_levels is not None:`
branches, so the typo only surfaces when the reform is instantiated
with that argument. Replace with `np.isin` so the reform doesn't
crash with AttributeError when exercised.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: vahid-ahmadi

changelog.d/257.md

@@ -0,0 +1 @@
+- Fix `AttributeError` in `cps.marriage_tax_reforms` when reform was instantiated with `child_education_levels` — replace nonexistent `np.is_in` (typo) with `np.isin`.

policyengine_uk/reforms/cps/marriage_tax_reforms.py

@@ -19,7 +19,7 @@ def formula(person, period):
                 child_meets_age_condition = person("age", period) <= max_child_age
                 return benunit.any(child_meets_age_condition)
             if child_education_levels is not None:
-                child_meets_education_condition = np.is_in(
+                child_meets_education_condition = np.isin(
                     person("education_level", period).decode_to_str(),
                     child_education_levels,
                 )
@@ -110,7 +110,7 @@ def formula(person, period):
                 child_meets_age_condition = person("age", period) <= max_child_age
                 return benunit.any(child_meets_age_condition)
             if child_education_levels is not None:
-                child_meets_education_condition = np.is_in(
+                child_meets_education_condition = np.isin(
                     person("education_level", period).decode_to_str(),
                     child_education_levels,
                 )