Use typing.Literal for mode

[adamchainz]

Is your feature request related to a problem? Please describe.

Only certain "mode" strings are accepted to Soup.find(), and underneath that, Soup._triage(). The type annotation str allows typos through for a runtime error.

Describe the solution you'd like

Use typing.Literal to specify the known mode names, so that such bugs can be caught at type checking type.

Describe alternatives you've considered
n/a

Additional context
n/a