test(test_git): get_tag_names should have List[Optional[str]] annotation

issue #285

tests/test_git.py

@@ -1,3 +1,6 @@
+import inspect
+from typing import List, Optional, Union
+
 import pytest
 
 from commitizen import git
@@ -68,3 +71,9 @@ def test_get_commits_author_and_email():
 
     assert commit.author is not ""
     assert "@" in commit.author_email
+
+
+def test_get_tag_names_has_correct_arrow_annotation():
+    arrow_annotation = inspect.getfullargspec(git.get_tag_names).annotations["return"]
+
+    assert arrow_annotation == List[Optional[str]]