Exclude conftest.py files from enforcing changed-code coverage (#6738)

* Exclude conftest.py files from enforcing changed-code coverage

Avoid spurious CI failure due to missing coverage of changed conftest.py lines.
conftest.py configures the pytest session.  It is tricky and not worthwhile to
ensure it is test-covered.

* Typo - missing comma

dev_tools/incremental_coverage.py

@@ -20,6 +20,7 @@
 from dev_tools import env_tools, shell_tools
 
 IGNORED_FILE_PATTERNS = [
+    r'^(.+/)?conftest\.py$',
     r'^dev_tools/.+',  # Environment-heavy code.
     r'^.+_pb2(_grpc)?\.py$',  # Auto-generated protobuf code.
     r'^(.+/)?setup\.py$',  # Installation code.