update tests and add code coverage with coverage, although coverage is not required

.coveragerc

@@ -0,0 +1,17 @@
+[report]
+exclude_lines =
+    # Skip @abstractmethod
+    @abstractmethod
+    @abc.abstractmethod
+
+    # Don't complain if tests don't hit defensive assertion code:
+    raise AssertionError
+    raise NotImplementedError
+
+    # Don't complain about missing debug-only code:
+    def __repr__
+    if self\.debug
+
+    # Don't complain if non-runnable code isn't run:
+    if 0:
+    if __name__ == .__main__.: