Suppress Graph deprecation warning in test suite

[kurtmckee]

Pull Request check-list

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• Do tests and lints pass with this change?
• Do the CI tests pass with this change (enable it first in your forked repo and wait for the github action build to finish)?
• Is the new or changed code fully tested?
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Description of change

When the test suite runs in CI, it shows that there are many uncaught DeprecationWarning messages (recent example).

These occur when Graph is instantiated and are expected. Therefore, they should be ignored by default, so other issues can be focused on.

This PR introduces the following changes:

• The pytest warnings configuration (currently present as the CLI argument -W always) is moved to pytest.ini
• The specific DeprecationWarning is now ignored by default in pytest.ini
• A new test was added to ensure that the DeprecationWarning is actually being thrown when Graph is instantiated.

If this merges, it will reduce the warning output that pytest displays after running the test suite.