fix(backend): fixed broken backends function after python3 migration

(grafted from 112c3403e5ce3301bd7e910fb76d49aeaa9d49a4)

rhodecode/config/environment.py

@@ -73,7 +73,7 b' def load_pyramid_environment(global_conf'
     load_rcextensions(root_path=settings_merged['here'])
 
     # Limit backends to `vcs.backends` from configuration, and preserve the order
-    for alias in rhodecode.BACKENDS.keys():
+    for alias in list(rhodecode.BACKENDS.keys()):
         if alias not in settings['vcs.backends']:
             del rhodecode.BACKENDS[alias]
 
@@ -81,7 +81,7 b' def load_pyramid_environment(global_conf'
                              key=lambda item: settings['vcs.backends'].index(item[0]))
     rhodecode.BACKENDS = collections.OrderedDict(_sorted_backend)
 
-    log.info('Enabled VCS backends: %s', rhodecode.BACKENDS.keys())
+    log.info('Enabled VCS backends: %s', list(rhodecode.BACKENDS.keys()))
 
     # initialize vcs client and optionally run the server if enabled
     vcs_server_uri = settings['vcs.server']