Fix clippy warnings

[andrewjcg]

warning: unexpected `cfg` condition name: `unwind`
  --> src/python_spy.rs:38:11
   |
38 |     #[cfg(unwind)]
   |           ^^^^^^ help: found config with similar value: `panic =
"unwind"`
   |
   = help: consider using a Cargo feature instead
   = help: or consider adding in `Cargo.toml` the `check-cfg` lint
config for the lint:
            [lints.rust]
            unexpected_cfgs = { level = "warn", check-cfg =
['cfg(unwind)'] }
   = help: or consider adding
`println!("cargo::rustc-check-cfg=cfg(unwind)");` to the top of the
`build.rs`
   = note: see
<https://doc.rust-lang.org/nightly/rustc/check-cfg/cargo-specifics.html>
for more information about checking conditional configuration