Silence pytest warnings about asyncio fixture scope (#6736)

In the current version of pytest (8.3.3) with the pytest-asyncio
module version 0.24.0, we see the following warnings at the beginning
of a pytest run:

```
warnings.warn(PytestDeprecationWarning(_DEFAULT_FIXTURE_LOOP_SCOPE_UNSET))

..../lib/python3.10/site-packages/pytest_asyncio/plugin.py:208:
PytestDeprecationWarning: The configuration option
"asyncio_default_fixture_loop_scope" is unset. The event loop scope for
asynchronous fixtures will default to the fixture caching scope. Future
versions of pytest-asyncio will default the loop scope for asynchronous
fixtures to function scope. Set the default fixture loop scope explicitly in
order to avoid unexpected behavior in the future. Valid fixture loop scopes
are: "function", "class", "module", "package", "session"
```

A [currently-open issue and discussion over in the pytest-asyncio
repo](pytest-dev/pytest-asyncio#924) suggests that
this is an undesired side-effect of a recent change in pytest-asyncio and is
not actually a significant warning. Moreover, the discussion suggests the
warning will be removed or changed in the future.

In the meantime, the warning is confusing because it makes it sound like
something is wrong. This simple PR silences the warning by adding a suitable
pytest init flag to `pyproject.toml'.

pyproject.toml

@@ -14,3 +14,5 @@ markers = [
     "slow: slow tests that should be skipped by default.",
     "weekly: tests to be run only by weekly CI automation.",
 ]
+# Silence deprecation warnings about option "asyncio_default_fixture_loop_scope"
+asyncio_default_fixture_loop_scope = "function"