Add Nunavut single status credit

changelog_entry.yaml

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+- bump: minor
+  changes:
+    changed:
+    - Nunavut Single Status Credit. 

policyengine_canada/parameters/gov/provinces/nu/tax/income/credits/single_status_credit/amount.yaml

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+description: Nunavut credits this amount under the single status credit.
+values:
+  2022-01-01: 16_000
+metadata:
+  unit: currency-CAD
+  label: Nunavut single status credit amount
+  reference:
+  - title: Nunavut Income Tax Act Part I Division B Subvision c 2.16
+    href: https://www.nunavutlegislation.ca/en/consolidated-law/income-tax-act-consolidation
+  - title: 2022 Form NU428, Nunavut Tax
+    href: https://www.canada.ca/content/dam/cra-arc/formspubs/pbg/5014-c/5014-c-22e.pdf#page=1

policyengine_canada/tests/gov/provinces/nu/tax/income/credits/single_status_credit/nu_single_status_credit.yaml

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+- name: Person with single status
+  period: 2023
+  input:
+    province_code: NU
+    is_married: false
+  output:
+    nu_single_status_credit: 16_000
+
+- name: Person with married status
+  period: 2023
+  input:
+    province_code: NU
+    is_married: true
+  output:
+    nu_single_status_credit: 0

policyengine_canada/variables/gov/provinces/nu/tax/income/credits/single_status_credit/single_status_credit.py

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+from policyengine_canada.model_api import *
+
+
+class nu_single_status_credit(Variable):
+    value_type = float
+    entity = Person
+    label = "Nunavut single status credit"
+    definition_period = YEAR
+    defined_for = ProvinceCode.NU
+
+    def formula(person, period, parameters):
+        single_status = ~person.household("is_married", period)
+        no_dependants = person.household("count_dependants", period) == 0
+        eligible = (single_status & no_dependants)
+        amount = parameters(
+            period
+        ).gov.provinces.nu.tax.income.credits.single_status_credit.amount
+        return amount * eligible