test: excluding NotImplementedError excluded too much

There are tests that mention NotImplementedError that were accidentally
excluded by this new metacov rule.  Instead, we'll manually pragma the
lines we want to exclude.

Author: nedbat

metacov.ini

@@ -44,9 +44,6 @@ exclude_lines =
     pragma: nested
     cov.stop\(\)
 
-    # Abstract methods
-    raise NotImplementedError
-
     # Lines that are only executed when we are debugging coverage.py.
     def __repr__
     pragma: debugging

tests/test_api.py

@@ -820,7 +820,7 @@ class IncludeOmitTestsMixin(UsingModulesMixin, CoverageTest):
     # An abstract method for subclasses to define, to appease mypy.
     def coverage_usepkgs(self, **kwargs_unused: TCovKwargs) -> Iterable[str]:
         """Run coverage on usepkgs, return a line summary. kwargs are for Coverage(**kwargs)."""
-        raise NotImplementedError()
+        raise NotImplementedError()     # pragma: not covered
 
     def filenames_in(self, summary: Iterable[str], filenames: str) -> None:
         """Assert the `filenames` are in the `summary`."""