Support new Mercurial password database
Created originally on Bitbucket by hstuartunity (Henrik Stuart)
Was already merged in Bitbucket before import, marked as merged by the import user
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.
This solves issue51.