Update tokio requirement from 0.1.11 to 0.3.7

Updates the requirements on [tokio](https://github.com/tokio-rs/tokio) to permit the latest version.
- [Release notes](https://github.com/tokio-rs/tokio/releases)
- [Commits](tokio-rs/tokio@tokio-0.1.11...tokio-0.3.7)

---
updated-dependencies:
- dependency-name: tokio
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

futures-util/Cargo.toml

@@ -47,7 +47,7 @@ pin-project-lite = "0.2.6"
 [dev-dependencies]
 futures = { path = "../futures", features = ["async-await", "thread-pool"] }
 futures-test = { path = "../futures-test" }
-tokio = "0.1.11"
+tokio = "0.3.7"
 
 [package.metadata.docs.rs]
 all-features = true

futures/Cargo.toml

@@ -30,7 +30,7 @@ assert_matches = "1.3.0"
 pin-project = "1.0.11"
 pin-utils = "0.1.0"
 static_assertions = "1"
-tokio = "0.1.11"
+tokio = "0.3.7"
 
 [features]
 default = ["std", "async-await", "executor"]