coverage: ignore def foo(): ... lines

These often appear in Protocols and ABCs and are never executed so not
expected to be covered.

Author: bluetech

.coveragerc

@@ -29,5 +29,6 @@ exclude_lines =
 
     ^\s*if TYPE_CHECKING:
     ^\s*@overload( |$)
+    ^\s*def .+: \.\.\.$
 
     ^\s*@pytest\.mark\.xfail