* Annotate the return type of as_sql for SpatialOperator.
Its subclasses inherits the type annotation from `SpatialOperator`, so
copying `as_sql` over is unnecessary.
Signed-off-by: Zixuan James Li <p359101898@gmail.com>
* Remove unnecessary as_sql definition.
`Query` inherits `as_sql` from `BaseExpression`, `GISLookup` inherits
`as_sql` from `Lookup`, and `BuiltinLookup` inherits `as_sql` from
`Lookup[_T]`. None is required to be redefined.
Signed-off-by: Zixuan James Li <p359101898@gmail.com>
* Unify return types of as_sql and friends as _AsSqlType.
`Tuple[str, _ParamsT]`, `Tuple[str, List[Union[str, int]]]` and other
similar type annotations are all replaced with the `_AsSqlType`
alias. Any as_sql definition that annotate the return type as `Any` is also
updated to use `_AsSqlType`.
Signed-off-by: Zixuan James Li <p359101898@gmail.com>