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Add a python3 license template. #11107 uses python3 exclusively so we need this.
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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
- [x] These changes do not require tests because covered by test-tidy self test
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Source-Repo: https://github.com/servo/servo
Source-Revision: 9d795c3a3eeb34ef1f380053954bbffb63e31386
6c94384a0d