add getformat (RustPython#3774)

Author: Miragecore

vm/src/builtins/float.rs

@@ -204,6 +204,24 @@ impl PyFloat {
         }
     }
 
+    #[pymethod(magic)]
+    fn getformat(&self, spec: PyStrRef, vm: &VirtualMachine) -> PyResult<String> {
+        if !matches!(spec.as_str(), "double" | "float") {
+            return Err(vm.new_value_error(
+                "__getformat__() argument 1 must be 'double' or 'float'".to_owned(),
+            ));
+        }
+
+        const BIG_ENDIAN: bool = cfg!(target_endian = "big");
+
+        Ok(if BIG_ENDIAN {
+            "IEEE, big-endian"
+        } else {
+            "IEEE, little-endian"
+        }
+        .to_owned())
+    }
+
     #[pymethod(magic)]
     fn abs(&self) -> f64 {
         self.value.abs()