Support Ruby 3

Besides a couple of [issues related to the new separation between
keyword and positional
arguments](https://www.ruby-lang.org/en/news/2019/12/12/separation-of-positional-and-keyword-arguments-in-ruby-3-0/),
this bug rails/rails#42098 is currently
hitting us. As we don't need to do anything from our side, we mark our
failing example so that we'll notice when it gets fixed upstream.

core/app/models/spree/return_item.rb

@@ -254,10 +254,9 @@ def validate_no_other_completed_return_items
       }).where.not(id: id).first
 
       if other_return_item && (new_record? || COMPLETED_RECEPTION_STATUSES.include?(reception_status.to_sym))
-        errors.add(:inventory_unit, :other_completed_return_item_exists, {
+        errors.add(:inventory_unit, :other_completed_return_item_exists,
           inventory_unit_id: inventory_unit_id,
-          return_item_id: other_return_item.id
-        })
+          return_item_id: other_return_item.id)
       end
     end
 

core/lib/spree/core/validators/email.rb

@@ -16,7 +16,7 @@ class EmailValidator < ActiveModel::EachValidator
 
     def validate_each(record, attribute, value)
       unless EMAIL_REGEXP.match? value
-        record.errors.add(attribute, :invalid, { value: value }.merge!(options))
+        record.errors.add(attribute, :invalid, **{ value: value }.merge!(options))
       end
     end
   end

core/spec/models/spree/store_credit_spec.rb

@@ -290,7 +290,13 @@
 
       subject { store_credit.validate_authorization(store_credit_attrs[:amount], store_credit.currency) }
 
-      it { is_expected.to be_truthy }
+      if RUBY_VERSION >= "3"
+        pending "https://github.com/rails/rails/issues/42098" do
+          it { is_expected.to be_truthy }
+        end
+      else
+        it { is_expected.to be_truthy }
+      end
     end
   end