Change cronet shebang to python3

Some of the cronet tests were failing cryptically and were also saying:

    vpython3 ./tools_unittest.py (11.73s) failed
    ../usr/bin/env: ‘python’: No such file or directory

It turns out that those were related. The api_static_checks_unittest.py
script was invoking update_api.py using os.system which uses the
shebang. The shebang referenced python, which is not installed on my
Linux system, causing the test failures. Changing to python3 fixed
everything.

Bug: 1309977
Change-Id: I0204c7d1763cf783e56b7dc44d321fe33c3c48c7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3621496
Reviewed-by: David Benjamin <davidben@chromium.org>
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>
Cr-Commit-Position: refs/heads/main@{#998516}