feat: Type of Income Paid on Tax Withholding Category

Custom Select income_type: the row of the withholding tax certificate
(50 tawi) a payment under the category is reported on, set for every
shipped category.

Frappe syncs fixtures before customizations on install and migrate, so a
fixture value for a Custom Field that does not exist yet is dropped;
before_install / before_migrate create the category fields first.

default_thai_company/tax_withholding.py

@@ -1,3 +1,6 @@
+import json
+import os
+
 import erpnext
 import frappe
 from erpnext.accounts.doctype.payment_entry.payment_entry import PaymentEntry
@@ -8,6 +11,7 @@ from erpnext.accounts.doctype.tax_withholding_category.tax_withholding_category
 	normal_round,
 )
 from frappe import _
+from frappe.modules.utils import sync_customizations_for_doctype
 from frappe.utils import cint, flt
 
 from default_thai_company.utils import money_in_words
@@ -122,6 +126,17 @@ def prepare_fixture_accounts(doc, method=None):
 			link_company(doc, company, ensure_company_accounts(company))
 
 
+def sync_category_customizations():
+	"""before_install / before_migrate: create the Tax Withholding Category Custom
+	Fields (custom/tax_withholding_category.json) ahead of the fixture import.
+	Frappe syncs fixtures before customizations, and a fixture value for a field
+	that does not exist yet is dropped."""
+	folder = frappe.get_app_path("default_thai_company", "default_thai_company", "custom")
+	filename = "tax_withholding_category.json"
+	with open(os.path.join(folder, filename)) as f:
+		sync_customizations_for_doctype(json.load(f), folder, filename)
+
+
 def company_ready(doc):
 	"""A Thai company whose chart of accounts exists."""
 	return doc.country == "Thailand" and frappe.db.exists("Account", {"company": doc.name})