Ticket #3871: fix

Index: twisted/conch/tap.py
===================================================================
--- twisted/conch/tap.py        (revision 26975)
+++ twisted/conch/tap.py        (working copy)
@@ -16,8 +16,13 @@
 from twisted.cred import portal
 from twisted.python import usage
 from twisted.application import strports
+try:
+    from twisted.cred import pamauth
+except ImportError:
+    pamauth = None
+else:
+    from twisted.cred.checkers import PluggableAuthenticationModulesChecker
 
-
 class Options(usage.Options):
     synopsis = "[-i <interface>] [-p <port>] [-d <dir>] "
     longdesc = "Makes a Conch SSH server."
@@ -36,8 +41,8 @@
     t.portal = portal.Portal(unix.UnixSSHRealm())
     t.portal.registerChecker(checkers.UNIXPasswordDatabase())
     t.portal.registerChecker(checkers.SSHPublicKeyDatabase())
-    if checkers.pamauth:
-        t.portal.registerChecker(checkers.PluggableAuthenticationModulesChecker())
+    if pamauth is not None:
+        t.portal.registerChecker(PluggableAuthenticationModulesChecker())
     t.dataRoot = config['data']
     t.moduliRoot = config['moduli'] or config['data']
     port = config['port']
Index: twisted/conch/checkers.py
===================================================================
--- twisted/conch/checkers.py   (revision 26975)
+++ twisted/conch/checkers.py   (working copy)
@@ -20,11 +20,6 @@
 except:
     shadow = None
 
-try:
-    from twisted.cred import pamauth
-except ImportError:
-    pamauth = None
-
 from zope.interface import implements, providedBy
 
 from twisted.conch import error