bpo-45101: Add consistency in usage message IO between 2 versions of python-config (GH-28162)

On --help output to stdout.
On error output to stderr.

Author: kiendang

Misc/NEWS.d/next/Tools-Demos/2021-09-05-02-47-48.bpo-45101.60Zqmt.rst

@@ -0,0 +1 @@
+Add consistency in usage message IO between 2 versions of python-config.

Misc/python-config.in

@@ -13,7 +13,8 @@ valid_opts = ['prefix', 'exec-prefix', 'includes', 'libs', 'cflags',
 
 def exit_with_usage(code=1):
     print("Usage: {0} [{1}]".format(
-        sys.argv[0], '|'.join('--'+opt for opt in valid_opts)), file=sys.stderr)
+        sys.argv[0], '|'.join('--'+opt for opt in valid_opts)),
+        file=sys.stdout if code == 0 else sys.stderr)
     sys.exit(code)
 
 try:

Misc/python-config.sh.in

@@ -4,7 +4,12 @@
 
 exit_with_usage ()
 {
-    echo "Usage: $0 --prefix|--exec-prefix|--includes|--libs|--cflags|--ldflags|--extension-suffix|--help|--abiflags|--configdir|--embed"
+    local USAGE="Usage: $0 --prefix|--exec-prefix|--includes|--libs|--cflags|--ldflags|--extension-suffix|--help|--abiflags|--configdir|--embed"
+    if [[ "$1" -eq 0 ]]; then
+        echo "$USAGE"
+    else
+        echo "$USAGE" >&2
+    fi
     exit $1
 }