Fix deprecation warnings

When testing with Python 3.12, there are several warnings. These are a mixture of:

• the use of pkg_resources; see also #233
• our tests using datetime.utcnow() instead of datetime.datetime.now(datetime.UTC)
• warnings in vendored packages that probably will disappear when they are upgraded

=============================== warnings summary ===============================
tests/conftest.py:10
  /home/runner/work/opensafely-cli/opensafely-cli/tests/conftest.py:10: DeprecationWarning: pkg_resources is deprecated as an API. See https://setuptools.pypa.io/en/latest/pkg_resources.html
    import pkg_resources

opensafely/_vendor/certifi/core.py:36
  /home/runner/work/opensafely-cli/opensafely-cli/opensafely/_vendor/certifi/core.py:36: DeprecationWarning: path is deprecated. Use files() instead. Refer to https://importlib-resources.readthedocs.io/en/latest/using.html#migrating-from-legacy for migration advice.
    _CACERT_CTX = get_path("opensafely._vendor.certifi", "cacert.pem")

opensafely/_vendor/google/protobuf/internal/well_known_types.py:91
  /home/runner/work/opensafely-cli/opensafely-cli/opensafely/_vendor/google/protobuf/internal/well_known_types.py:91: DeprecationWarning: datetime.datetime.utcfromtimestamp() is deprecated and scheduled for removal in a future version. Use timezone-aware objects to represent datetimes in UTC: datetime.datetime.fromtimestamp(timestamp, datetime.UTC).
    _EPOCH_DATETIME_NAIVE = datetime.datetime.utcfromtimestamp(0)

tests/test_codelists.py::test_codelists_update
tests/test_codelists.py::test_codelists_update
  /home/runner/work/opensafely-cli/opensafely-cli/opensafely/codelists.py:89: DeprecationWarning: datetime.datetime.utcnow() is deprecated and scheduled for removal in a future version. Use timezone-aware objects to represent datetimes in UTC: datetime.datetime.now(datetime.UTC).
    "downloaded_at": f"{datetime.datetime.utcnow()}Z",

tests/test_init.py::test_should_version_check
tests/test_init.py::test_should_version_check
  /home/runner/work/opensafely-cli/opensafely-cli/opensafely/__init__.py:45: DeprecationWarning: datetime.datetime.utcnow() is deprecated and scheduled for removal in a future version. Use timezone-aware objects to represent datetimes in UTC: datetime.datetime.now(datetime.UTC).
    four_hours_ago = datetime.utcnow() - timedelta(hours=4)

tests/test_init.py::test_should_version_check
  /home/runner/work/opensafely-cli/opensafely-cli/tests/test_init.py:15: DeprecationWarning: datetime.datetime.utcnow() is deprecated and scheduled for removal in a future version. Use timezone-aware objects to represent datetimes in UTC: datetime.datetime.now(datetime.UTC).
    timestamp = (datetime.utcnow() - timedelta(hours=5)).timestamp()

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
================== 233 passed, 8 warnings in 63.59s (0:01:03) ==================

Start by rerunning the tests with Python 3.12 and inspect the warnings in tests/conftest.py, opensafely/codelists.py, opensafely/init.py, and tests/test_init.py. Check #233 for the pkg_resources work and distinguish project warnings from those in opensafely/_vendor. Done means the actionable project warnings are addressed and the test suite passes without them.