test(refactor): abstract method compatible with both pytest and mypy

Using Protocol as a base class introduces a __init__ method, which
pytest does not like.  This way keeps both tools happy.

metacov.ini

@@ -62,6 +62,7 @@ exclude_lines =
     # Not-real code for type checking
     if TYPE_CHECKING:
     class .*\(Protocol\):
+    pragma: for typing
 
     # OS error conditions that we can't (or don't care to) replicate.
     pragma: cant happen

tests/test_api.py

@@ -15,7 +15,7 @@
 import sys
 import textwrap
 
-from typing import cast, Callable, Dict, Iterable, List, Optional, Protocol, Set
+from typing import cast, Callable, Dict, Iterable, List, Optional, Set
 
 import pytest
 
@@ -814,19 +814,14 @@ def test_bug_572(self) -> None:
             cov.report()
 
 
-class CoverageUsePkgs(Protocol):
-    """A number of test classes have the same helper method."""
-    def coverage_usepkgs(
-        self,
-        **kwargs: TCovKwargs,
-    ) -> Iterable[str]:
+class IncludeOmitTestsMixin(UsingModulesMixin, CoverageTest):
+    """Test methods for coverage methods taking include and omit."""
+
+    # An abstract method for subclasses to define, to appease mypy.
+    def coverage_usepkgs(self, **kwargs_unused: TCovKwargs) -> Iterable[str]:  # pragma: for typing
         """Run coverage on usepkgs, return a line summary. kwargs are for Coverage(**kwargs)."""
         return ""
 
-
-class IncludeOmitTestsMixin(CoverageUsePkgs, UsingModulesMixin, CoverageTest):
-    """Test methods for coverage methods taking include and omit."""
-
     def filenames_in(self, summary: Iterable[str], filenames: str) -> None:
         """Assert the `filenames` are in the `summary`."""
         for filename in filenames.split():