switch to 2021 edition

Author: wfraser

Cargo.toml

@@ -2,7 +2,7 @@
 name = "dropbox-sdk"
 version = "0.19.0-beta2"
 authors = ["Bill Fraser <[email protected]>"]
-edition = "2018"
+edition = "2021"
 description = "Rust bindings to the Dropbox API, generated by Stone from the official spec."
 categories = ["api-bindings"]
 keywords = ["dropbox", "sdk", "cloud", "storage"]