tests/data/README.md: Use 'python3' executable instead of 'python'
'python' may be Python 2, according to https://peps.python.org/pep-0394/#for-python-runtime-distributors
Migrated from D8422 (view on Phabricator)
Staging instance, all changes can be removed at any time
'python' may be Python 2, according to https://peps.python.org/pep-0394/#for-python-runtime-distributors
Migrated from D8422 (view on Phabricator)