[travis] use PHP 8.0 to patch return types and run deps=low

Author: nicolas-grekas

DeprecationErrorHandler/Deprecation.php

@@ -82,7 +82,9 @@ public function __construct($message, array $trace, $file)
                 $this->message = $parsedMsg['deprecation'];
                 $this->originClass = $parsedMsg['class'];
                 $this->originMethod = $parsedMsg['method'];
-                $this->originalFilesStack = $parsedMsg['files_stack'];
+                if (isset($parsedMsg['files_stack'])) {
+                    $this->originalFilesStack = $parsedMsg['files_stack'];
+                }
                 // If the deprecation has been triggered via
                 // \Symfony\Bridge\PhpUnit\Legacy\SymfonyTestsListenerTrait::endTest()
                 // then we need to use the serialized information to determine

Legacy/SymfonyTestsListenerTrait.php

@@ -260,7 +260,7 @@ public function endTest($test, $time)
             unlink($this->runsInSeparateProcess);
             putenv('SYMFONY_DEPRECATIONS_SERIALIZE');
             foreach ($deprecations ? unserialize($deprecations) : [] as $deprecation) {
-                $error = serialize(['deprecation' => $deprecation[1], 'class' => $className, 'method' => $test->getName(false), 'triggering_file' => isset($deprecation[2]) ? $deprecation[2] : null]);
+                $error = serialize(['deprecation' => $deprecation[1], 'class' => $className, 'method' => $test->getName(false), 'triggering_file' => isset($deprecation[2]) ? $deprecation[2] : null, 'files_stack' => isset($deprecation[3]) ? $deprecation[3] : []]);
                 if ($deprecation[0]) {
                     // unsilenced on purpose
                     trigger_error($error, \E_USER_DEPRECATED);