Don't special-case an isascii implementation for Python 3.6

Python 3.7 is now our minimum-supported Python, and so we don't need to add
our own version of isascii any more.

See #2566.

rich/rule.py

@@ -51,13 +51,9 @@ class Rule(JupyterMixin):
     ) -> RenderResult:
         width = options.max_width
 
-        # Python3.6 doesn't have an isascii method on str
-        isascii = getattr(str, "isascii", None) or (
-            lambda s: all(ord(c) < 128 for c in s)
-        )
         characters = (
             "-"
-            if (options.ascii_only and not isascii(self.characters))
+            if (options.ascii_only and not self.characters.isascii())
             else self.characters
         )