DateTimePatch: keep exceptions as class attributes

We have code depending on these attributes, they were used in pattern
like this:

```py
try:
  DateTime(user_input)
except DateTime.DateError:
  error("invalid date")
```

so it's safer to keep the attributes for now, since we are already
patching a lot

product/ERP5Type/patches/DateTimePatch.py

@@ -278,3 +278,11 @@ def DateTime_parse(self, st, datefmt=getDefaultDateFormat()):
 
 DateTimeKlass._parse = DateTime_parse
 
+
+# DateTime 3 removed exceptions as class attributes (since
+# zopefoundation/DateTime commit 8114618 ), but we have some code expecting
+# these attributes, so undo this patch for convenience.
+DateTimeKlass.DateTimeError = DateTimeError
+DateTimeKlass.SyntaxError = SyntaxError
+DateTimeKlass.DateError = DateError
+DateTimeKlass.TimeError = TimeError