Do not consider paths with a single leading slash as absolute on Windows

This is consistent with the new "os.path.isabs" behavior since Python 3.13

picard/util/__init__.py

@@ -268,7 +268,15 @@ def is_absolute_path(path):
     """Similar to os.path.isabs, but properly detects Windows shares as absolute paths
     See https://bugs.python.org/issue22302
     """
-    return os.path.isabs(path) or (IS_WIN and os.path.normpath(path).startswith("\\\\"))
+    if IS_WIN:
+        # Two backslashes indicate a UNC path.
+        if path.startswith("\\\\"):
+            return True
+        # Consider a single slash at the start not relative. This is the default
+        # for `os.path.isabs` since Python 3.13.
+        elif path.startswith("\\") or path.startswith("/"):
+            return False
+    return os.path.isabs(path)
 
 
 def samepath(path1, path2):

test/test_utils.py

@@ -400,6 +400,7 @@ def test_remove_windows_drive(self):
 
 class IsAbsolutePathTest(PicardTestCase):
 
+    @unittest.skipIf(IS_WIN, "POSIX test")
     def test_is_absolute(self):
         self.assertTrue(is_absolute_path('/foo/bar'))
         self.assertFalse(is_absolute_path('foo/bar'))
@@ -410,7 +411,11 @@ def test_is_absolute(self):
     def test_is_absolute_windows(self):
         self.assertTrue(is_absolute_path('D:/foo/bar'))
         self.assertTrue(is_absolute_path('D:\\foo\\bar'))
-        self.assertTrue(is_absolute_path('\\foo\\bar'))
+        self.assertFalse(is_absolute_path('\\foo\\bar'))
+        self.assertFalse(is_absolute_path('/foo/bar'))
+        self.assertFalse(is_absolute_path('foo/bar'))
+        self.assertFalse(is_absolute_path('./foo/bar'))
+        self.assertFalse(is_absolute_path('../foo/bar'))
         # Paths to Windows shares
         self.assertTrue(is_absolute_path('\\\\foo\\bar'))
         self.assertTrue(is_absolute_path('\\\\foo\\bar\\'))