Adds a python formatter

[zanderso]

This PR wires up a python formatter and formats all the flutter/engine python files according to Google open source style with a couple of small tweaks to be consistent with the Flutter recipes repo.

This is pre-work for adding a python linter, since pylint has opinions about formatting, which will be superseded by this actual formatting checker.

[flutter-dashboard]

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[skia-gold]

Gold has detected about 3 new digest(s) on patchset 2.
View them at https://flutter-engine-gold.skia.org/cl/github/33797

[flutter-dashboard]

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Changes reported for pull request #33797 at sha 8e03b78