Auto merge of #24575 - marmeladema:issue-23607/compat, r=SimonSapin

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

---
<!-- 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

<!-- Either: -->
- [x] There are tests for these changes

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etc/taskcluster/decision_task.py

@@ -205,13 +205,13 @@ def linux_tidy_unit():
         .with_treeherder("Linux x64", "Tidy+Unit")
         .with_script("""
             ./mach test-tidy --no-progress --all
-            ./mach build --dev
-            ./mach test-unit
-            ./mach package --dev
-            ./mach build --dev --features canvas2d-raqote
-            ./mach build --dev --features layout-2020
-            ./mach build --dev --libsimpleservo
-            ./mach build --dev -p servo-gst-plugin
+            python3 ./mach build --dev
+            python3 ./mach test-unit
+            python3 ./mach package --dev
+            python3 ./mach build --dev --features canvas2d-raqote
+            python3 ./mach build --dev --features layout-2020
+            python3 ./mach build --dev --libsimpleservo
+            python3 ./mach build --dev -p servo-gst-plugin
             ./mach test-tidy --no-progress --self-test
 
             ./etc/memory_reports_over_time.py --test