remove unmaintained atty crate from dependencies

There's a replacement in std that requires 1.70 which is old enough for us.

Cargo.toml

@@ -12,12 +12,11 @@ readme = "README.md"
 
 [package.metadata]
 # Keep this at least 1 year old.
-# (or not... 1.75 is required for "-> impl Trait" sadly)
+# 1.75 is required for "-> impl Trait"
 msrv = "1.75.0" # Dec 28, 2023
 
 [dependencies]
 async-lock = "3.3.0"
-atty = "0.2.14"
 base64 = "0.22"
 bytes = "1.6.0"
 log = "0.4"

src/oauth2.rs

@@ -13,7 +13,7 @@
 //! [OAuth types summary]: https://developers.dropbox.com/oauth-guide#summary
 
 use std::env;
-use std::io::{self, Write};
+use std::io::{self, IsTerminal, Write};
 use std::sync::Arc;
 use async_lock::RwLock;
 use base64::Engine;
@@ -664,7 +664,7 @@ pub fn get_auth_from_env_or_prompt() -> Authorization {
         }
     }
 
-    if !atty::is(atty::Stream::Stdin) {
+    if !io::stdin().is_terminal() {
         panic!("DBX_CLIENT_ID and/or DBX_OAUTH not set, and stdin not a TTY; cannot authorize");
     }