Specifically pin pyproject.toml as pytest config source

This is needed to fix the coverage failures detailed at
nedbat/coveragepy#512

We may instead need to have this listed somewhere else however, as
if pytest can't pick up the pyproject.toml it can't read the line
telling it to use it.

pyproject.toml

@@ -32,7 +32,13 @@ requires = ["setuptools>=61.0"]
 build-backend = "setuptools.build_meta"
 
 [tool.pytest.ini_options]
-addopts = ["--import-mode=importlib", "--verbose", "--cov", "--cov-append"]
+addopts = [
+    "--import-mode=importlib",
+    "--verbose",
+    "--cov",
+    "--cov-append",
+    "--cov-config=pyproject.toml",
+]
 filterwarnings = ['ignore::DeprecationWarning:pytest']
 minversion = "7"
 pythonpath = ["src"]