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

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- [x] There are tests for these changes

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etc/taskcluster/docker/base.dockerfile

@@ -14,11 +14,17 @@ RUN \
         git \
         ca-certificates \
         #
-        # Running mach
+        # Running mach with Python 2
         python \
         python-pip \
         python-dev \
         #
+        # Running mach with Python 3
+        python3 \
+        python3-pip \
+        python3-dev \
+        virtualenv \
+        #
         # Compiling C modules when installing Python packages in a virtualenv
         gcc \
         #