Include files for coverage instead of omitting others

pyproject.toml:
Use specific include for files to use in the coverage report instead of
relying on a list of omitted files (which may grow over time and not
cover all use-cases).

pyproject.toml

@@ -21,7 +21,7 @@ exclude = '''
 source = ["libkeyringctl"]
 
 [tool.coverage.report]
-omit = ["tests/*", ".tox/*"]
+include = ["libkeyringctl/*", "keyringctl"]
 precision = 2
 show_missing = true