3p/nanopb: Specify python3 instead of python
3p/nanopb: Specify python3 instead of python

The script requires Python 3, so run it with python3 instead of python
in case this isn't run in a virtual environment.

Change-Id: I787f3bcd85ce2113ed0b67b88399dea5ef2246ed
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/52865
Pigweed-Auto-Submit: Wyatt Hepler <cca3bedb37979eacd157a9bceb473582df8aea1e@google.com>
Reviewed-by: Rob Mohr <d07614a8a0b466db08f68224f87d738b0f9615cf@google.com>
Commit-Queue: Auto-Submit <14637eda5603879df170705285bf30f5996692f0@pigweed.google.com.iam.gserviceaccount.com>

google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed