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.

src/yubikey.rs

@@ -190,12 +190,7 @@ impl YubiKey {
     /// [`yubikey::reader::Context`][`Context`] to select from the available
     /// PC/SC readers.
     pub fn open() -> Result<Self> {
-        let mut readers = Context::open().map_err(|e| match e {
-            Error::PcscError {
-                inner: Some(pcsc::Error::NoReadersAvailable),
-            } => Error::NotFound,
-            other => other,
-        })?;
+        let mut readers = Context::open()?;
         let mut reader_iter = readers.iter()?;
 
         if let Some(reader) = reader_iter.next() {
@@ -213,12 +208,7 @@ impl YubiKey {
 
     /// Open a YubiKey with a specific serial number.
     pub fn open_by_serial(serial: Serial) -> Result<Self> {
-        let mut readers = Context::open().map_err(|e| match e {
-            Error::PcscError {
-                inner: Some(pcsc::Error::NoReadersAvailable),
-            } => Error::NotFound,
-            other => other,
-        })?;
+        let mut readers = Context::open()?;
 
         for reader in readers.iter()? {
             let yubikey = match reader.open() {