Redirect warning output from stderr to stdout in doctests

src/sage/doctest/forker.py

@@ -261,7 +261,8 @@
     r"""
     Displays a warning message with a traceback.
 
-    INPUT: see :func:`warnings.showwarning`.
+    INPUT: see :func:`warnings.showwarning` with the difference that with ``file=None``
+           the message will be written to stdout.
 
     OUTPUT: none
 
@@ -293,7 +294,7 @@
     lines.extend(traceback.format_exception_only(category, category(message)))
 
     if file is None:
-        file = sys.stderr
+        file = sys.stdout
     try:
         file.writelines(lines)
         file.flush()
@@ -2193,7 +2194,7 @@
         sage: W.start()
         sage: DC = DocTestController(DD, filename)
         sage: reporter = DocTestReporter(DC)
         sage: W.join()  # Wait for worker to finish
         sage: result = W.result_queue.get()
         sage: reporter.report(FDS, False, W.exitcode, result, "")
             [... tests, ...s wall]
@@ -2320,7 +2321,7 @@
             ....: except OSError:
             ....:     print("Write end of pipe successfully closed")
             Write end of pipe successfully closed
             sage: W.join()  # Wait for worker to finish
         """
         super().start()
 
@@ -2356,7 +2357,7 @@
             sage: W.start()
             sage: while W.rmessages is not None:
             ....:     W.read_messages()
             sage: W.join()
             sage: len(W.messages) > 0
             True
         """
@@ -2388,7 +2389,7 @@
             sage: FDS = FileDocTestSource(filename, DD)
             sage: W = DocTestWorker(FDS, DD)
             sage: W.start()
             sage: W.join()
             sage: W.save_result_output()
             sage: sorted(W.result[1].keys())
             ['cputime', 'err', 'failures', 'optionals', 'tests', 'walltime', 'walltime_skips']