Fix pagetemplate import for Plone 4.3 compatibility.

`zope.app.pagetemplate.viewpagetemplatefile.ViewPageTemplateFile` was moved to `zope.browserpage.viewpagetemplatefile.ViewPageTemplateFile`

`zope.app.pagetemplate` is no longer included in Plone >= 4.3
`zope.browserpage.viewpagetemplatefile` works from Plone >= 4.1