allow -i 3.13t (#2324)

Related to #2315 

As of `maturin` 1.7.5 we accept `maturin build -i python3.13t` for the
free-threaded build, it seems like we might want to allow `maturin build
-i 3.13t` (and assume CPython).

src/build_options.rs

@@ -1266,12 +1266,7 @@ fn find_interpreter_in_sysconfig(
             )
         } else if let Some(ver) = python.strip_prefix("python") {
             // Also accept things like `python3.13t` for free-threaded python
-            let (ver, abiflags) =
-                if let Some(ver) = ver.strip_prefix('-').unwrap_or(ver).strip_suffix('t') {
-                    (ver, "t")
-                } else {
-                    (ver, "")
-                };
+            let (ver, abiflags) = maybe_free_threaded(ver.strip_prefix('-').unwrap_or(ver));
             (InterpreterKind::CPython, ver, abiflags)
         } else if python
             .chars()
@@ -1280,7 +1275,8 @@ fn find_interpreter_in_sysconfig(
             .unwrap_or(false)
         {
             // Eg: -i 3.9 without interpreter kind, assume it's CPython
-            (InterpreterKind::CPython, &*python, "")
+            let (ver, abiflags) = maybe_free_threaded(&python);
+            (InterpreterKind::CPython, ver, abiflags)
         } else {
             // if interpreter not known
             if std::path::Path::new(&python).is_file() {
@@ -1319,6 +1315,14 @@ fn find_interpreter_in_sysconfig(
     Ok(interpreters)
 }
 
+fn maybe_free_threaded(python_ver: &str) -> (&str, &str) {
+    if let Some(ver) = python_ver.strip_suffix('t') {
+        (ver, "t")
+    } else {
+        (python_ver, "")
+    }
+}
+
 /// We need to pass the global flags to cargo metadata
 /// (https://github.com/PyO3/maturin/issues/211 and https://github.com/PyO3/maturin/issues/472),
 /// but we can't pass all the extra args, as e.g. `--target` isn't supported, so this tries to