Raise minimum pcsc version to remove workaround (#478)

In iqlusioninc/yubikey.rs#88 we added a workaround for what turned out
to be a bug in `pcsc`, where an error was returned if no readers were
available, instead of returning an empty iterator. `pcsc 2.3.1` was
published in 2019, so we can safely rely on it.

Cargo.toml

@@ -35,7 +35,7 @@ num-integer = "0.1"
 pbkdf2 = { version = "0.11", default-features = false }
 p256 = "0.11"
 p384 = "0.11"
-pcsc = "2"
+pcsc = "2.3.1"
 rand_core = { version = "0.6", features = ["std"] }
 rsa = "0.7"
 secrecy = "0.8"