bug symfony#30677 Fix type hints for email attachments (fabpot)

This PR was merged into the 4.3-dev branch.

Discussion
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Fix type hints for email attachments

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | yes
| New feature?  | no <!-- don't forget to update src/**/CHANGELOG.md files -->
| BC breaks?    | no     <!-- see https://symfony.com/bc -->
| Deprecations? | no <!-- don't forget to update UPGRADE-*.md and src/**/CHANGELOG.md files -->
| Tests pass?   | yes    <!-- please add some, will be required by reviewers -->
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

`createDataPart()` must return a `DataPart` instance, so `attachPart()` must only accept `DataPart` instances and `getAttachments()` is guaranteed to return an array of `DataPart` instances.

Commits
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8059c50 fixed type hints for email attachments

Author: fabpot

src/Symfony/Component/Mime/Email.php

@@ -372,15 +372,15 @@ public function embedFromPath(string $path, string $name = null, string $content
     /**
      * @return $this
      */
-    public function attachPart(AbstractPart $part)
+    public function attachPart(DataPart $part)
     {
         $this->attachments[] = ['part' => $part];
 
         return $this;
     }
 
     /**
-     * @return AbstractPart[]
+     * @return DataPart[]
      */
     public function getAttachments(): array
     {