OK, thanks for reporting it.  A couple of things. I'm not sure why the PyQt5 build is looking for the MacOSX10.10.sdk as you report.  There do seem to be some issues building a universal Python when using a current default Xcode SDK; I'm going to leave this issue open to investigate those.  You can get around those issues by using --enable-universalsdk=/ which uses the header files and libraries installed in traditional locations by the Command Line Tools rather than from the SDK locations.  Also, as documented in the Mac README (https://hg.python.org/cpython/file/3.5/Mac/README) where universal builds are described, --with-universal-archs=64-bit is a legacy configuration option that is only supported with an OS X 10.5 SDK, because it includes ppc64 which is no longer supported by current Xcode compilers.  On current systems, building with no --with-universal-archs option specified will produce 64-bit Intel binaries.  Finally, depending on what your needs are, it might be easier to install PyQt5 and Python 3.5 from a third-party distributor, like Homebrew, MacPorts, or conda.