Remove undocumented stderr parameter

subprocess.check_call(args, stderr=True) merges child stderr into parent
stdout instead of separating streams.

CPython accepts it because bool inherits from int, interpreting True as
file descriptor 1 (stdout). This redirects child stderr to parent stdout,
mixing diagnostic output with normal output.

This removes stderr parameter to use default (stderr=None), which inherits
parent stderr per subprocess documentation, properly separating stdout and
stderr streams.

Author: jserv

runtest.py

@@ -22,7 +22,7 @@ def _notebook_run(path):
                 "--allow-errors",
                 "--ExecutePreprocessor.timeout=60",
                 "--output", fout.name, path]
-        subprocess.check_call(args, stderr=True)
+        subprocess.check_call(args)
 
         fout.seek(0)
         nb = nbformat.read(fout, nbformat.current_nbformat)