Migrate python invocations to python3 (#87759)

Python 2 has been end-of-life'd since January 2020, all Python scripts
on which Flutter depends have been migrated to Python 3. This changes
documentation and remaining invocations to explicitly invoke python3.

Issue: https://github.com/flutter/flutter/issues/83043

dev/bots/README.md

@@ -47,7 +47,7 @@ To run `prepare_package.dart` locally:
 
 - Make sure the `depot_tools` is in your `PATH`. If you're on Windows, you also need
   an environment variable called `DEPOT_TOOLS` with the path to `depot_tools` as value.
-- Run `gsutil.py config` (or `python %DEPOT_TOOLS%\gsutil.py` on Windows) to
+- Run `gsutil.py config` (or `python3 %DEPOT_TOOLS%\gsutil.py` on Windows) to
   authenticate with your auth token.
 - Create a local temp directory. `cd` into it.
 - Run `dart [path to your normal Flutter repo]/dev/bots/prepare_package.dart

dev/bots/prepare_package.dart

@@ -669,7 +669,7 @@ class ArchivePublisher {
     }
     if (platform.isWindows) {
       return _processRunner.runProcess(
-        <String>['python', path.join(platform.environment['DEPOT_TOOLS']!, 'gsutil.py'), '--', ...args],
+        <String>['python3', path.join(platform.environment['DEPOT_TOOLS']!, 'gsutil.py'), '--', ...args],
         workingDirectory: workingDirectory,
         failOk: failOk,
       );

dev/bots/test/prepare_package_test.dart

@@ -289,7 +289,7 @@ void main() {
       late FakeProcessManager processManager;
       late Directory tempDir;
       final String gsutilCall = platform.isWindows
-          ? 'python ${path.join("D:", "depot_tools", "gsutil.py")}'
+          ? 'python3 ${path.join("D:", "depot_tools", "gsutil.py")}'
           : 'gsutil.py';
       final String releasesName = 'releases_$platformName.json';
       final String archiveName = platform.isLinux ? 'archive.tar.xz' : 'archive.zip';