Suppress a coverage error

Author: jwodder

tox.ini

@@ -58,7 +58,9 @@ addopts =
     # to some tests spawning subprocesses with changed working directories.
     --cov-config=tox.ini
     --no-cov-on-fail
-filterwarnings = error
+filterwarnings =
+    error
+    ignore:.*No source for code:coverage.exceptions.CoverageWarning
 markers =
     describe_exclude: Tests that use `git describe --exclude` (Added in Git 2.13.0)
     oldsetup: Tests to only run under pre-v64 setuptools