Make tools/gn compatible with Python 3

[cbracken]

Python 2.7 was end-of-lifed on 2020-01-01. This makes tools/gn
compatible with both Python 2.7 and Python 3.

One in a series of fixes for flutter/flutter#83043

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[cbracken]

/cc @godofredoc, since I'm not sure what the plan for Python migration on the bots is.

[godofredoc]

/cc @godofredoc, since I'm not sure what the plan for Python migration on the bots is.

python in the bots is provided as a cipd package and added to the path. The difference is that we may need to explicitly use python3 to run them instead of just python.