Fix `smart_repr()` to also clean upper-case hex addresses.

Windows Pythons seem to like printing addresses in upper-case, while Linux
Pythons like lower-case hexes.

This led to an amusing (for a given value of "amusing", anyway) situation
where some repr tests would fail if the objects they were testing happened
to be allocated at an address with a hex digit in the range A..F.

rest_framework/utils/representation.py

@@ -40,7 +40,7 @@ def smart_repr(value):
     # <django.core.validators.RegexValidator object at 0x1047af050>
     # Should be presented as
     # <django.core.validators.RegexValidator object>
-    value = re.sub(' at 0x[0-9a-f]{4,32}>', '>', value)
+    value = re.sub(' at 0x[0-9A-Fa-f]{4,32}>', '>', value)
 
     return value