Use repr in fromisoformat error message

This reverses commit 67b74a98b2 per Serhiy Storchaka's suggestion:

     I suggested to use %R in the error message because including the raw
     string can be confusing in the case of empty string, or string
     containing trailing whitespaces, invisible or unprintable characters.

We agree that it is better to change both the C and pure Python versions
to use repr.

Author: pganssle

Lib/datetime.py

@@ -857,7 +857,7 @@ def fromisoformat(cls, date_string):
             assert len(date_string) == 10
             return cls(*_parse_isoformat_date(date_string))
         except Exception:
-            raise ValueError('Invalid isoformat string: %s' % date_string)
+            raise ValueError(f'Invalid isoformat string: {date_string!r}')
 
 
     # Conversions to string
@@ -1369,7 +1369,7 @@ def fromisoformat(cls, time_string):
         try:
             return cls(*_parse_isoformat_time(time_string))
         except Exception:
-            raise ValueError('Invalid isoformat string: %s' % time_string)
+            raise ValueError(f'Invalid isoformat string: {time_string!r}')
 
 
     def strftime(self, fmt):
@@ -1646,13 +1646,13 @@ def fromisoformat(cls, date_string):
         try:
             date_components = _parse_isoformat_date(dstr)
         except ValueError:
-            raise ValueError('Invalid isoformat string: %s' % date_string)
+            raise ValueError(f'Invalid isoformat string: {date_string!r}')
 
         if tstr:
             try:
                 time_components = _parse_isoformat_time(tstr)
             except ValueError:
-                raise ValueError('Invalid isoformat string: %s' % date_string)
+                raise ValueError(f'Invalid isoformat string: {date_string!r}')
         else:
             time_components = [0, 0, 0, 0, None]