Improve pytest warning filtering

[llucax]

What's needed?

We need to easily filter out protobuf version warnings.

Proposed solution

[tool.pytest.ini_options]
addopts = "-vv"
filterwarnings = [
  "error",
  "once::DeprecationWarning",
  "once::PendingDeprecationWarning",
  # We use a raw string (single quote) to avoid the need to escape special
  # chars as this is a regex
  'ignore:Protobuf gencode version .*exactly one major version older.*:UserWarning',
]

Additional context

• frequenz-floss/frequenz-client-reporting-python@114af31#r2104417626