[meta] Drupal 9 support

Problem/Motivation

With #3042774: Drupal 9 Deprecated Code Report committed, the work to get the module fully compatible with Drupal 9 is not done yet.

There are currently 106 test failures on D9:
https://www.drupal.org/pift-ci-job/1655810

I've scanned through the report, and I found the following kind of errors:

• Exception: Drupal\Tests\BrowserTestBase::$defaultTheme is required.

• Drupal\Tests\feeds\Unit\FeedExpireHandlerTest::testPostExpire
  Drupal\Core\DependencyInjection\ContainerNotInitializedException: \Drupal::$container is not initialized yet. \Drupal::setContainer() must be called with a real container.

• Drupal\Tests\feeds\Kernel\Entity\FeedTest::testPushImport
  Error: Class 'Doctrine\Common\Inflector\Inflector' not found

• Drupal\Tests\feeds\Unit\Component\CsvParserTest::testInvalidFilePath
  The @expectedException, @expectedExceptionCode, @expectedExceptionMessage, and @expectedExceptionMessageRegExp annotations are deprecated. They will be removed in PHPUnit 9. Refactor your test to use expectException(), expectExceptionCode(), expectExceptionMessage(), or expectExceptionMessageRegExp() instead.

Proposed resolution

See child issues for each identified error.

Remaining tasks

• #3095636: Add defaultTheme in tests extending BrowserTestBase.
• #3129113: Replace @expectedException with expectException() call
• #3129114: Fix Drupal 9 test failures
• #3136594: Drupal 9 Deprecated Code Report Strikes Back

User interface changes

Hopefully none.

API changes

Hopefully none.

Data model changes

Hopefully none.