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/piv.rs

@@ -55,7 +55,7 @@ use crate::{
 use elliptic_curve::sec1::EncodedPoint as EcPublicKey;
 use log::{debug, error, warn};
 use rsa::{BigUint, RsaPublicKey};
-use std::{convert::TryFrom, str::FromStr};
+use std::str::FromStr;
 
 #[cfg(feature = "untested")]
 use {