Auto merge of #28419 - dominiccooney:py3bat, r=jdm

Make mach.bat request Python 3

The mach Python script no longer supports Python 2, so don't use
Python 2.

Fixes #28379

Signed-off-by: Dominic Cooney <dominic.cooney@gmail.com>

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Make mach.bat request Python 3

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `___` with appropriate data: -->
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #28379 (GitHub issue number if applicable)

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- [ ] There are tests for these changes OR
- [X] These changes do not require tests because this is the script which you use to launch the tests

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mach.bat

@@ -4,5 +4,5 @@ where /Q py.exe
 IF %ERRORLEVEL% NEQ 0 (
   python mach %*
 ) ELSE (
-  py -2 mach %*
+  py -3 mach %*
 )