fix: require python 3.7+

[gcf-owl-bot]

Source-Link: googleapis/synthtool@4f89b13
Post-Processor: gcr.io/cloud-devrel-public-resources/owlbot-python:latest@sha256:e7bb19d47c13839fe8c147e50e02e8b6cf5da8edd1af8b82208cd6f66cc2829c

[shollyman]

prerelease failure doesn't seem to find the system test.

nox > py.test tests/system ============================= test session starts ============================== platform linux -- Python 3.8.12, pytest-7.1.2, pluggy-1.0.0 rootdir: /tmpfs/src/github/python-bigquery-reservation plugins: asyncio-0.18.3, cov-3.0.0 asyncio: mode=legacy collected 0 items =============================== warnings summary =============================== .nox/prerelease_deps-3-8/lib/python3.8/site-packages/pytest_asyncio/plugin.py:191 /tmpfs/src/github/python-bigquery-reservation/.nox/prerelease_deps-3-8/lib/python3.8/site-packages/pytest_asyncio/plugin.py:191: DeprecationWarning: The 'asyncio_mode' default value will change to 'strict' in future, please explicitly use 'asyncio_mode=strict' or 'asyncio_mode=auto' in pytest configuration file. config.issue_config_time_warning(LEGACY_MODE, stacklevel=2) -- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html ============================== 1 warning in 0.01s ============================== ERROR: file or directory not found: tests/system nox > Command py.test tests/system failed with exit code 4 nox > Session prerelease_deps-3.8 failed. cleanup