doc: update status of Python 3 support
Refs: https://github.com/nodejs/TSC/pull/781#issuecomment-559666380 Based on the completion of the Python 3 strategic initiative, update statements around Python 3 support.
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
documentation is changed or added -
commit message follows commit guidelines