Add an explicit ASCII check.

It may seem like this isn't necessary, because:

    datetime.date.fromisoformat('1963-06-1৪')

fails (properly! it's non-ASCII) on CPython, but that's only because the
datetime module swaps itself out for the C implementation, and the C
implementation blows up on the non-ASCII string.

The pure-python implementation (which in some situations may get used)
does no check itself for ASCII-ness.

There's a comment saying as much here:
https://github.com/python/cpython/blob/bb3e0c240bc60fe08d332ff5955d54197f79751c/Lib/datetime.py#L266-L267

So, let's check explicitly ourselves regardless of this working
occasionally.

Thanks PyPy for pointing out the bug.

Author: Julian

jsonschema/_format.py

@@ -364,7 +364,7 @@ def is_regex(instance):
 def is_date(instance):
     if not isinstance(instance, str):
         return True
-    return _is_date(instance)
+    return instance.isascii() and datetime.date.fromisoformat(instance)
 
 
 @_checks_drafts(draft3="time", raises=ValueError)