gitlab: Implement download of promotion artifact

arch_release_promotion/gitlab.py:
Add `download_promotion_artifact()` to allow download of a project
release's promotion artifact to a directory.

tests/test_gitlab.py:
Add tests for `download_promotion_artifact()`.

arch_release_promotion/gitlab.py

 from pathlib import Path
 from typing import List, Optional
+from urllib import request
 
 import gitlab
 
@@ -126,6 +127,46 @@ class Upstream(gitlab.Gitlab):
 
         return artifact_zip
 
+    def download_promotion_artifact(self, tag_name: str, temp_dir: Path) -> Path:
+        """Download the promotion artifact of a project's release
+
+        Parameters
+        ----------
+        tag_name: str
+            The tag_name of the release to download
+        temp_dir: Path
+            The directory into which to download
+
+        Returns
+        -------
+        Path
+            The file path of the downloaded file
+        """
+
+        project = self.projects.get(self.name)
+        artifact_links: List[str] = []
+
+        for release in project.releases.list():
+            # only select releases that are promoted
+            if release.tag_name == tag_name and any(link.name == "Promotion artifact" for link in release.links.list()):
+                artifact_links += [link.url for link in release.links.list() if link.name == "Promotion artifact"]
+
+        if not artifact_links:
+            raise RuntimeError(
+                f"There is no promotion artifact to download for the release '{tag_name}' of project '{project.name}'."
+            )
+        if len(artifact_links) > 1:
+            raise RuntimeError(
+                f"There is more than one promotion artifact to download for the release '{tag_name}' "
+                f"of project '{project.name}'. Something is wrong!"
+            )
+
+        print(f"Downloading promotion artifact of release '{tag_name}' for '{self.name}'...")
+        filename, headers = request.urlretrieve(artifact_links[0], filename=temp_dir / Path("promotion.zip"))
+        print("Done!")
+
+        return Path(filename)
+
     def promote_release(self, tag_name: str, file: str) -> None:
         """Upload a promotion file to the project and add it as a link to a release
 

tests/test_gitlab.py

+from contextlib import nullcontext as does_not_raise
 from pathlib import Path
-from typing import Optional
+from typing import ContextManager, Optional
 from unittest.mock import Mock, patch
 
 from gitlab.exceptions import GitlabGetError
-from pytest import mark
+from pytest import mark, raises
 
 from arch_release_promotion import gitlab
 
@@ -129,6 +130,75 @@ def test_gitlab_download_release() -> None:
     upstream.download_release(tag_name="0.1.0", temp_dir=Path("/tmp"), job_name="job")
 
 
+@mark.parametrize(
+    "releases_available, tag_name, multi_promotion, has_link, link_name, link_url, expectation",
+    [
+        (True, "0.1.0", False, True, "Promotion artifact", "https://foo.bar/download/this/file.zip", does_not_raise()),
+        (True, "0.1.0", False, True, "Foo artifact", "https://foo.bar/download/this/file.zip", raises(RuntimeError)),
+        (
+            False,
+            "0.1.0",
+            False,
+            True,
+            "Promotion artifact",
+            "https://foo.bar/download/this/file.zip",
+            raises(RuntimeError),
+        ),
+        (
+            True,
+            "0.1.0",
+            True,
+            True,
+            "Promotion artifact",
+            "https://foo.bar/download/this/file.zip",
+            raises(RuntimeError),
+        ),
+    ],
+)
+@patch("arch_release_promotion.gitlab.request.urlretrieve")
+def test_gitlab_download_promotion_artifact(
+    urlretrieve_mock: Mock,
+    releases_available: bool,
+    tag_name: str,
+    multi_promotion: bool,
+    has_link: bool,
+    link_name: str,
+    link_url: str,
+    expectation: ContextManager[str],
+) -> None:
+    urlretrieve_mock.return_value = ("foo", None)
+    upstream = gitlab.Upstream(
+        url="https://foo.bar-mc.foo",
+        private_token="THISISAFAKETOKEN",
+        name="foo/bar",
+    )
+    link = Mock()
+    link.name = link_name
+    link.url = link_url
+    links = Mock()
+    links.list.return_value = [link]
+
+    release = Mock()
+    release.links = links
+    release.tag_name = tag_name
+
+    releases = Mock()
+    releases.list.return_value = [release] if releases_available else []
+    if releases_available and multi_promotion:
+        releases.list.return_value = [release, release]
+
+    project = Mock()
+    project.releases = releases
+
+    projects = Mock()
+    projects.get.return_value = project
+    upstream.projects = projects
+
+    with expectation:
+        upstream.download_promotion_artifact(tag_name="0.1.0", temp_dir=Path("/tmp"))
+        urlretrieve_mock.assert_called_once_with(link_url, filename=Path("/tmp") / Path("promotion.zip"))
+
+
 @mark.parametrize("same_tag_name", [(True), (False)])
 def test_gitlab_promote_release(same_tag_name: bool) -> None:
     upstream = gitlab.Upstream(