Hide obnoxious ruamel.yaml warnings during test runs.

setup.cfg

@@ -5,7 +5,7 @@ description_file=README.md
 testpaths = tests
 addopts = --cov-report term-missing:skip-covered --cov=borgmatic --ignore=tests/end-to-end
 filterwarnings = 
-    ignore:Coverage disabled.*:pytest.PytestWarning
+    ignore:Deprecated call to `pkg_resources.declare_namespace\('ruamel'\)`.*:DeprecationWarning
 
 [flake8]
 ignore = E501,W503