PHP 8.0/8.1 compatibility for Feeds 8.x-3.x

Hello

Collaborating with @joelpittet on this for ContributionWeekend2022

Watched over @joelpittet's shoulder while barking out the occasional "helpful" comment.

I closed merge request !46 because there has been a new release of Pathauto which makes it no longer necessary to require the dev version of Pathauto for testing.

Opened a new issue fork to actively work on tests that fail on PHP 8.1. Some PHP 8.1 issues have been fixed in the mean time in other issues:
#3266706: Deprecated getIterator() and count() using PHP 8.1
#3267244: Deprecated function: trim(): Passing null to parameter #1 ($string) of type string is deprecated in Drupal\feeds\Plugin\Field\FieldType\FeedsItem->preSave()
#3266704: PHP 8.1: do not pass null to Html::escape() and Unicode::truncate().

UpdateCustomSourcesUpdateTest test failure

The following failure looks like a bug in Drupal Core:

Drupal\Tests\feeds\Functional\Update\UpdateCustomSourcesUpdateTest::testUpdateCustomSources
Exception: Deprecated function: mb_strtolower(): Passing null to parameter #1 ($string) of type string is deprecated

mb_strtolower() is called in
In \Drupal\Core\Config\Entity\Query\Condition::compile():

elseif (!is_bool($condition['value'])) {
  $condition['value'] = mb_strtolower($condition['value']);
}

I've inspected what the condition is and it happens to be:
Array ( [field] => uuid [value] => NULL [operator] => = [langcode] => NULL)
In \Drupal\Core\Config\Entity\ConfigEntityBase::preSave() the entity's UUID is passed as condition:

$matching_entities = $storage->getQuery()
  ->condition('uuid', $this->uuid())
  ->execute();

But it is valid that ::uuid() can return NULL, see \Drupal\Core\Entity\EntityBase::uuid():

/**
 * {@inheritdoc}
 */
public function uuid() {
  return $this->uuid ?? NULL;
}

I've found some existing core issues related to this:
https://www.drupal.org/project/issues/drupal?text=mb_strtolower+null+uui...

I think it would be good to open a core issue for above.

auto_detect_line_endings

Using ini_set('auto_detect_line_endings', '1'); is deprecated in PHP 8.1. According to https://php.watch/versions/8.1/auto_detect_line_endings-ini-deprecated, this means that line-endings for \r characters are no longer supported.
But removing the usage of that in \Drupal\feeds\Component\CsvParser does now cause a test failure.
So the question is: should Feeds actively remove support for CSV files with \r line endings or is there a workaround?

Hopefully fixed all test failures now:

• PasswordTest was failing because one of the imported items failed validation, but %guid was NULL and passing that as an argument t() causes an error when the translation gets rendered:
  

  Deprecated function: htmlspecialchars(): Passing null to parameter #1 ($string) of type string is deprecated in Drupal\Component\Utility\Html::escape() (line 424 of core/lib/Drupal/Component/Utility/Html.php).

  Drupal\Component\Utility\Html::escape(NULL) (Line: 262)
  Drupal\Component\Render\FormattableMarkup::placeholderEscape(NULL) (Line: 232)
  Drupal\Component\Render\FormattableMarkup::placeholderFormat('Entity with %guid failed to validate', Array) (Line: 195)
  Drupal\Core\StringTranslation\TranslatableMarkup->render() (Line: 15)
  Drupal\Core\StringTranslation\TranslatableMarkup->__toString() (Line: 1)

• UpdateCustomSourcesUpdateTest gets fixed by adding a 'uuid' value to the test config file "feeds.feed_type.csv.yml" in tests/fixtures/feeds-8.x-3.0-alpha11.
• CsvParserTest gets fixed by actively removing support for CSV files with \r line endings - the test coverage for this gets removed.

I merged the changes. I looked once more on why for certain methods of \Drupal\feeds\Component\CsvParser I added #[\ReturnTypeWillChange] and that is because the return type "mixed" is only available since PHP 8.1. Hm, should add an additional commit that notes that.