YDA-5603: fix email encoding issue in DR module

Emails regarding preregistration approvals to researchers failed due
to a wrong body encoding.

datarequest.py

@@ -2679,7 +2679,7 @@ def mail_datarequest_approved_researcher(ctx, truncated_title, researcher_email,
                      cc=cc,
                      actor=user.full_name(ctx),
                      subject=(u"YOUth data request {} (\"{}\") (data assessment only): approved".format(request_id, truncated_title) if dao else "YOUth data request {} (\"{}\"): preregistration approved".format(request_id, truncated_title)),
-                     body="""Dear {},
+                     body=u"""Dear {},
 
 The preregistration of your data request has been approved. The YOUth data manager will now create a Data Transfer Agreement for you to sign. You will be notified when it is ready.