Add migration info for MakeModal

git-svn-id: http://svn.wxwidgets.org/svn/wx/wxPython@73871 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Small tweak and tests to allow using objects supporting only the old buffer interface to be used as buffer sources.

git-svn-id: http://svn.wxwidgets.org/svn/wx/wxPython@73872 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Keep GetParser public

git-svn-id: http://svn.wxwidgets.org/svn/wx/wxPython@73873 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775