tests: pass re.MULTILINE to re.sub as 'flags' - not in 'count' position stable

tests: pass re.MULTILINE to re.sub as 'flags' - not in 'count' position

This bug was caught by the new Python 3.13 warning:

  DeprecationWarning: 'count' is passed as positional argument

--- a/tests/test-doctest.py	Thu Jun 29 20:02:27 2023 +0200
+++ b/tests/test-doctest.py	Thu Jan 11 21:58:55 2024 +0100
@@ -21,9 +21,11 @@
             r'''^mercurial\.\w+\.(\w+): (['"])(.*?)\2''',
             r'\1: \3',
             got2,
-            re.MULTILINE,
+            flags=re.MULTILINE,
         )
-        got2 = re.sub(r'^mercurial\.\w+\.(\w+): ', r'\1: ', got2, re.MULTILINE)
+        got2 = re.sub(
+            r'^mercurial\.\w+\.(\w+): ', r'\1: ', got2, flags=re.MULTILINE
+        )
         return any(
             doctest.OutputChecker.check_output(self, w, g, optionflags)
             for w, g in [(want, got), (want2, got2)]