bpo-25008: Deprecate smtpd and point to aiosmtpd (python#274)

* bpo-25008: Deprecate smtpd and point to aiosmtpd.

* Simplify the aiosmtpd URL.

Author: warsaw

Doc/library/smtpd.rst

@@ -13,6 +13,12 @@
 
 This module offers several classes to implement SMTP (email) servers.
 
+.. seealso::
+
+    The `aiosmtpd <http://aiosmtpd.readthedocs.io/>`_ package is a recommended
+    replacement for this module.  It is based on :mod:`asyncio` and provides a
+    more straightforward API.  :mod:`smtpd` should be considered deprecated.
+
 Several server implementations are present; one is a generic
 do-nothing implementation, which can be overridden, while the other two offer
 specific mail-sending strategies.

Misc/NEWS

@@ -735,6 +735,9 @@ C API
 Documentation
 -------------
 
+- bpo-25008: Document smtpd.py as effectively deprecated and add a pointer to
+  aiosmtpd, a third-party asyncio-based replacement.
+
 - Issue #26355: Add canonical header link on each page to corresponding major
   version of the documentation. Patch by Matthias Bussonnier.