Use Pytest's skipif decorator instead of returning to skip assert sta…

…tements.

tests/util/test_platform.py

@@ -1,3 +1,4 @@
+import pytest
 import platform
 
 from keyring.util.platform_ import (
@@ -10,19 +11,17 @@
 )
 
 
+@pytest.mark.skipif(platform.system() != "Linux", reason="Requires platform.system() == 'Linux'")
 def test_platform_Linux():
     # rely on the Github Actions workflow to run this on different platforms
-    if platform.system() != "Linux":
-        return
 
     assert config_root == _config_root_Linux
     assert data_root == _data_root_Linux
 
 
+@pytest.mark.skipif(platform.system() != "Windows", reason="Requires platform.system() == 'Windows'")
 def test_platform_Windows():
     # rely on the Github Actions workflow to run this on different platforms
-    if platform.system() != "Windows":
-        return
 
     assert config_root == _config_root_Windows
     assert data_root == _data_root_Windows