Add Python-ref cloning GILOnceCell::<Py<T>>::clone_ref

[jakelishman]

This adds a helper method to GILOnceCell containing Py pointers, allowing a cheap clone by Python-space incref. This might want to be supplanted/supplemented by a derive macro as part of #4133.

Close #4428

As an aside: I'm on an Intel Mac and wasn't able to run nox -s clippy-all due to pre-existing failures in some sort of macro expansion such as:

nox > cargo clippy '--features=full multiple-pymethods' --all-targets --workspace -- --deny=warnings

error: custom attribute panicked
   --> src/coroutine.rs:125:1
    |
125 | #[pymethods(crate = "crate")]
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = help: message: Unrecognized literal: `c"__name__"`
    = note: this error originates in the macro `crate::inventory::submit` which comes from the expansion of the attribute macro `pymethods` (in Nightly builds, run with -Z macro-backtrace for more info)

I assume this is what you were seeing in CI prior to #3985.

[davidhewitt]

Thanks, this seems a reasonable short-term addition even if the future for GILOnceCell is unclear...

[davidhewitt]

Not sure about your local clippy issue. Does running cargo update resolve it?