Enable some mach commands to be run with python3
This change finally enable the following commands to be run with python3:
* `build`
* `test-unit`
* `package`
As previously explained, `test-tidy` will require more work in the wpt repository directly. Maybe `test-tidy --no-wpt` is achievable relatively quickly though.
For possible remaining bits that might need to be worked on, see https://github.com/servo/servo/issues/23607
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<!-- 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
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- [x] There are tests for these changes
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