Add `Strideable` conformance, remove slice initializer. (#22849)
Add `Strideable` conformance, remove slice initializer. (#22849)

Add `Strideable` conformance to `PythonObject`.
This enables `stride(from:to:by:)` and `stride(from:through:by:)`
with `PythonObject`.

Remove unnecessary `init(sliceStart:end:step:)` initializer.
Instead, directly call the builtin `Python.slice()`.

Note: conditionally conforming `StrideTo` and `StrideThrough` to
`PythonConvertible` would enable indexing into `PythonObject` arrays
using them. However, this is difficult because `_stride` is an internal
stored property in `StrideTo` and `StrideThrough`.

pvieito/PythonKit