`update-signatures` fails with some tartufo configs

[jhall1-godaddy]

🐛 Bug Report

It cannot parse a config like this:

[tool.tartufo]
exclude-signatures = [
    "abc",
    "def",
]

To Reproduce

run tartufo update-signatures . on a local repo with a config like the above.

Expected Behavior

The config should be handled regardless of whether it's a list of strings, or a list of dictionaries.