Let's try branch coverage

Author: Tinche

Diff for: pyproject.toml

@@ -102,6 +102,7 @@ msgspec = [
 addopts = "-l --benchmark-sort=fullname --benchmark-warmup=true --benchmark-warmup-iterations=5  --benchmark-group-by=fullname"
 
 [tool.coverage.run]
+branch = true
 parallel = true
 source_pkgs = ["cattrs", "tests"]