Support new Mercurial password database

From Mercurial version 3.9 (changed in 2c019aac6b99), the password manager
no longer derives from HTTPPasswordMgrWithDefaultRealm where hgkeyring hooks
in, but rather uses a password database on the password manager object.

mercurial_keyring.py

@@ -303,9 +303,13 b' class HTTPPasswordHandler(object):'
             return url_user, url_passwd, self.SRC_URL, base_url
 
         # Extract data from urllib (in case it was already stored)
-        urllib_user, urllib_pwd \
-            = urllib2.HTTPPasswordMgrWithDefaultRealm.find_user_password(
-                pwmgr, realm, authuri)
+        if isinstance(pwmgr, urllib2.HTTPPasswordMgrWithDefaultRealm):
+            urllib_user, urllib_pwd = \
+                urllib2.HTTPPasswordMgrWithDefaultRealm.find_user_password(
+                    pwmgr, realm, authuri)
+        else:
+            urllib_user, urllib_pwd = pwmgr.passwddb.find_user_password(
+                realm, authuri)
         if urllib_user and urllib_pwd:
             return urllib_user, urllib_pwd, self.SRC_URLCACHE, base_url