servo: Merge #12650 - Add python3 licence check template (from shinglyu:py3-tidy); r=Wafflespeanut

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Add a python3 license template. #11107 uses python3 exclusively so we need this.
---
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- [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

servo/python/tidy/servo_tidy/licenseck.py

@@ -31,6 +31,14 @@ licenses = [
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 """,
 
+"""\
+#!/usr/bin/env python3
+
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+""",
+
 """\
 // This Source Code Form is subject to the terms of the Mozilla Public
 // License, v. 2.0. If a copy of the MPL was not distributed with this