refactor: simplify conditional logic

Command: `sourcery review --fix --enable de-morgan .`
diff --git a/erpnext/accounts/party.py b/erpnext/accounts/party.py
index 1efe35c..008614e 100644
--- a/erpnext/accounts/party.py
+++ b/erpnext/accounts/party.py
@@ -775,7 +775,7 @@
 				frozen_accounts_modifier = frappe.db.get_single_value(
 					"Accounts Settings", "frozen_accounts_modifier"
 				)
-				if not frozen_accounts_modifier in frappe.get_roles():
+				if frozen_accounts_modifier not in frappe.get_roles():
 					frappe.throw(_("{0} {1} is frozen").format(party_type, party_name), PartyFrozen)
 
 		elif party_type == "Employee":