Added depr warning for the outdated cherrypy server adapter.

If you are using this adapter, simply switch to 'cheroot'
This should fix some recent and some very old issues regarding cherrypy:

fix #947 Leave explicit the maxima version supported the CherryPy (<= 9.0.0)
fix #932 Add ServerAdapter (fix CherryPy ServerAdapter)
fix #685 Update CherryPy SSL to use latest API and work on Py3
fix #574 Allow custom bind_addr for CherryPy

(backported from commit be90814)
[juergh: Adjust context, drop modifications of test/travis-requirements.txt
 which does not exist in 0.12.]
Signed-off-by: Juerg Haefliger <juergh@protonmail.com>

bottle.py

@@ -2793,7 +2793,11 @@ class server_cls(server_cls):
 
 class CherryPyServer(ServerAdapter):
     def run(self, handler): # pragma: no cover
-        from cherrypy import wsgiserver
+        depr(0, 13, "The wsgi server part of cherrypy was split into a new "
+                    "project called 'cheroot'.", "Use the 'cheroot' server "
+                    "adapter instead of cherrypy.")
+        from cherrypy import wsgiserver # This will fail for CherryPy >= 9
+
         self.options['bind_addr'] = (self.host, self.port)
         self.options['wsgi_app'] = handler