Fix str.isascii

Lib/test/test_unicode.py

@@ -679,8 +679,6 @@ class UnicodeTest(string_tests.CommonTest,
         self.assertFalse('\U0001F40D'.isalpha())
         self.assertFalse('\U0001F46F'.isalpha())
 
-    # TODO: RUSTPYTHON
-    @unittest.expectedFailure
     def test_isascii(self):
         super().test_isascii()
         self.assertFalse("\u20ac".isascii())

vm/src/obj/objstr.rs

@@ -795,7 +795,7 @@ impl PyString {
 
     #[pymethod]
     fn isascii(&self) -> bool {
-        !self.value.is_empty() && self.value.chars().all(|c| c.is_ascii())
+        self.value.chars().all(|c| c.is_ascii())
     }
 
     #[pymethod]