Exclude Sphinx test roots to avoid duplicate conf module collisions

These directories are Sphinx fixture projects, not importable package code.

- Both tests/roots/test-altairplot/conf.py and tests/roots/test-altairplot-filearg/conf.py are top-level conf.py files.
- When mypy walks tests, it maps both to the same module name (conf) and raises Duplicate module named "conf".
- These files are test data for Sphinx builds (integration fixtures), so type-checking them adds little value compared to checking sphinxext_altair/ and actual test modules.

So the exclude avoids a structural false-positive from fixture layout, not a real typing bug.

Author: joelostblom

pyproject.toml

@@ -297,6 +297,9 @@ split-on-trailing-comma = false
 [tool.ruff.lint.per-file-ignores]
 "!sphinxext_altair/altairplot.py" = ["ANN"]
 
+[tool.mypy]
+exclude = "^tests/roots/"
+
 [[tool.mypy.overrides]]
 module = ["altair.*"]
 ignore_missing_imports = true