sagemathgh-39647: Redirect warning output from stderr to stdout in doctests
    
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Pytest considers ouput in `stderr` as errors, thus we redirect the
doctest warnings to `stdout` instead. Doesn't seem to make a difference
for sage's doctest runner.

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URL: sagemath#39647
Reported by: Tobias Diez
Reviewer(s):

Author: Release Manager

src/sage/doctest/forker.py

@@ -261,7 +261,8 @@ def showwarning_with_traceback(message, category, filename, lineno, file=None, l
     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 @@ def showwarning_with_traceback(message, category, filename, lineno, file=None, l
     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()