tomcli set PATH append [OPTIONS] SELECTOR VALUE could accept an arbitrary number of values to append at the same time, similar to tomcli set PATH list [OPTIONS] SELECTOR [VALUE].... This would offer a small ergonomic and performance benefit when appending multiple values.

[bar]
bat = ["baz"]

tomcli set foo.toml append bar.bat biff

[bar]
bat = ["baz", "b", "i", "f", "f"]

I expected bat = ["baz", "biff"].

Since the tests are included in the PyPI sdist, please consider including tox.ini as well (by adding the line include tox.ini to MANIFEST.in). This would make it easier to install the test dependencies and would ensure the definition for the slow_test mark is available to pytest.