Fix compilation errors on latest nightly

`proc-macro2` before 1.0.60 is incompatible with the latest nightly
since an unstable feature conditionally used by `proc-macro2` was
renamed.

Fixed with a targeted `cargo update`:

```console
$ cargo update --package proc-macro2
    Updating crates.io index
    Updating proc-macro2 v1.0.56 -> v1.0.63
```

See also:

- rust-lang/rust#113152
- dtolnay/proc-macro2#391
- artichoke/artichoke#2619

Author: lopopolo