#12283: Fixed regression in smtplib quoting of leading dots in DATA.

I unfortunately introduced the regression when I refactored the code,
and there were no tests of quoting so it wasn't caught.  Now there
is one.
diff --git a/Misc/NEWS b/Misc/NEWS
index c5d2cec..dab5355 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -22,6 +22,8 @@
 Library
 -------
 
+- Issue #12283: Fixed regression in smtplib quoting of leading dots in DATA.
+
 - Issue #12168: SysLogHandler now allows NUL termination to be controlled using
   a new 'append_nul' attribute on the handler.