Add config to error when warnings raised during tests

[dstansby]

In general it's good practice to raise an error in tests when a warning is emitted by code. This can be done by adding the following to pyproject.toml:

[tool.pytest.ini_options]
filterwarnings = [
    "error",
]

This serves several purposes:

• Catches any deprecation warnings in upstream dependencies, allowing code to be modified in advance of it becoming an actual error
• Make sure any warnings emitted by the package are intended.

pytest has tools to catch or filter warnings where they are intended: https://docs.pytest.org/en/stable/how-to/capture-warnings.html