Cleanup: declare __all__ for Python scripts

Declare all to make public public API's explicit and
help detect unused code.

tests/coverage/coverage.py

@@ -3,6 +3,10 @@
 #
 # SPDX-License-Identifier: GPL-2.0-or-later
 
+__all__ = (
+    "main",
+)
+
 import argparse
 import os
 import shutil