2021 edition upgrade; MSRV 1.56 (#343)

Changes the `edition` to 2021 in both the `yubikey` and `yubikey-cli`
crates.

Removes `TryFrom`/`TryInto` imports, now that they're in the prelude.

src/transaction.rs

@@ -11,7 +11,6 @@ use crate::{
     Buffer, ObjectId,
 };
 use log::{error, trace};
-use std::convert::TryInto;
 use zeroize::Zeroizing;
 
 #[cfg(feature = "untested")]