Omit 'if TYPE_CHECKING' blocks from coverage

These blocks only exist to create code which evaluates under mypy
but does not run at runtime. As a result, they are impossible to cover
with `coverage` on a testsuite, and make sense to exclude from
coverage reporting.

.coveragerc

@@ -3,3 +3,7 @@
 branch = True
 source = jsonschema
 omit = */jsonschema/_reflect.py,*/jsonschema/__main__.py,*/jsonschema/benchmarks/*,*/jsonschema/tests/fuzz_validate.py
+
+[report]
+exclude_lines =
+    if TYPE_CHECKING: