Update README not to specify the wrong type for the SMTP server

The README file had said that the SMTP server would be an instance of
smtpd.SMTPServer, but that's actually no longer true now that we don't
use the smtpd module anymore. In this commit I'm updating that bit of
documentation not to specifically reference smtpd.SMTPServer. Instead,
it's a bit vague about what kind of SMTP server is provided, only
promising an API "similar to smtpd.SMTPServer". This gives us freedom
to change the exact API offered by that class if necessary.

README.rst

@@ -132,7 +132,8 @@ poking around in the code itself.
     is the same as ``httpserver`` only with SSL encryption.
 
 ``smtpserver``
-    provides a threaded instance of ``smtpd.SMTPServer`` running on localhost.
+    provides a threaded SMTP server, with an API similar to ``smtpd.SMTPServer``,
+    (the deprecated class from the Python standard library) running on localhost.
     It has the following attributes:
 
     * ``addr`` - server address as tuple (host as str, port as int)