deps: update libp2p-core requirement from 0.39 to 0.40

Updates the requirements on [libp2p-core](https://github.com/libp2p/rust-libp2p) to permit the latest version.
- [Release notes](https://github.com/libp2p/rust-libp2p/releases)
- [Changelog](https://github.com/libp2p/rust-libp2p/blob/master/CHANGELOG.md)
- [Commits](libp2p/rust-libp2p@v0.39.0...libp2p-core-v0.40.0)

---
updated-dependencies:
- dependency-name: libp2p-core
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

Cargo.toml

@@ -12,7 +12,7 @@ authors = ["umgefahren <[email protected]>"]
 arti-client = { version = "0.8", default-features = false }
 async-std-crate = { package = "async-std", version = "1", optional = true, default-features = false }
 futures = "0.3"
-libp2p-core = { version = "0.39" }
+libp2p-core = { version = "0.40" }
 thiserror = "1"
 tokio-crate = { package = "tokio", version = "1", optional = true, default-features = false }
 tor-rtcompat = "0.8"