This reworks the internals of the UNSET symbol (which was introduced in

Djblets 3.3) and makes it easier to integrate it with typing logic and

type checkers.
UnsetSymbol is now an Enum, allowing UNSET to be defined as a

Literal. This works along with a new Unsettable[_T] type alias to

mark whether a parameter or variable allows UNSET to be set.
By making this a Literal, type checkers can type narrow when a code

compares a value to UNSET, avoiding any type errors that still wanted

to consider UnsetSymbol a possible value.

Made use of this in some in-progress code. Verified that mypy and pyright

successfully narrowed the types when comparing against UNSET.