Drupal 9 readiness

Thanks for the issue and the patch, but from my understanding because we're using the deprecated interface we're explicity not Drupal 9 compatible and Drupal 8 compatible? We'd need a Drupal 9 branch that removes the deprecated interface use.

Ran drupal-check against bb665be. Here is an error not listed in the patch above.

 ------ ----------------------------------------------------------------
  Line   tests/src/Kernel/ImageOptimizePipelineTest.php
 ------ ----------------------------------------------------------------
  184    Call to deprecated function file_default_scheme():
         in drupal:8.8.0 and is removed from drupal:9.0.0. Use
         \Drupal::config('system.file')->get('default_scheme') instead.
  203    Call to deprecated function file_default_scheme():
         in drupal:8.8.0 and is removed from drupal:9.0.0. Use
         \Drupal::config('system.file')->get('default_scheme') instead.
 ------ ----------------------------------------------------------------

Looks good for me. All Drupal deprecations covered and key added in info file.

The service "entity.manager" is deprecated too. See #2894261: Deprecated service entity.manager needs to be replaced with entity_type.manager in ContentEntityForm

+++ b/src/ImageAPIOptimizeProcessorInterface.php
@@ -17,7 +16,7 @@ use Drupal\Component\Plugin\PluginInspectionInterface;
-interface ImageAPIOptimizeProcessorInterface extends PluginInspectionInterface, ConfigurableInterface, DependentPluginInterface, ConfigurablePluginInterface {
+interface ImageAPIOptimizeProcessorInterface extends PluginInspectionInterface, ConfigurableInterface, DependentPluginInterface {

These lines represent an API break, so we'll hold off on that and do a quick version 3 that's Drupal 8/9 compatible I reckon.

Works for me
One slight change is needed - depending on whether the new release version is set to support Drupal 8.8.2 or above only, the test module needs the core version removed, or if supporting below Drupal 8.8.2, a specific core_version_requirements line added.

Patched in 12 into 10. The decision was already made as a part of that patch.

Committed to 8.x-3.x.

@Steven Jones reckon we can switch to semver if its going to be a new major version?

(it will need to be a 4.x now, since 8.x-3.x was created..)

Yeah, sorry about that!

Making the switch now.