Allow readonly property write in __unserialize

Fixes phpstan/phpstan#6384

src/Rules/Properties/ReadOnlyPropertyAssignRule.php

@@ -67,7 +67,7 @@ public function processNode(Node $node, Scope $scope): array
 				throw new ShouldNotHappenException();
 			}
 
-			if (strtolower($scopeMethod->getName()) === '__construct') {
+			if (strtolower($scopeMethod->getName()) === '__construct' || strtolower($scopeMethod->getName()) === '__unserialize') {
 				if (!$scope->getType($propertyFetch->var) instanceof ThisType) {
 					$errors[] = RuleErrorBuilder::message(sprintf('Readonly property %s::$%s is not assigned on $this.', $declaringClass->getDisplayName(), $propertyReflection->getName()))->build();
 				}

tests/PHPStan/Rules/Properties/data/readonly-assign.php

@@ -164,3 +164,23 @@ public function doFoo(Foo $foo, int $i)
 	}
 
 }
+
+class Unserialization
+{
+
+	private readonly int $foo;
+
+	public function __construct(int $foo)
+	{
+		$this->foo = $foo; // constructor - fine
+	}
+
+	/**
+	 * @param array<int, int> $data
+	 */
+	public function __unserialize(array $data) : void
+	{
+		[$this->foo] = $data; // __unserialize - fine
+	}
+
+}