I own bug #780354.  The problem is that socket.makefile() no
longer returns a true file.  This breaks code that had assumed
it did.  The marshal module is one such instance.

In the above bug's corresponce Martin stated:

    Instead, I think the C modules that expect file objects need
    to be fixed.

Later on Martin stated that he thought it would be fine to simply
document the limitation for the marshal module.  I ignored that
and fixed it in the attached patch.  It includes modified docs and
an updated test suite.

It's obviously somewhat slower than the current version when
reading/writing true files.  That's to be expected.  I wanted to
get it right first.  I imagine something can be done to speed
things up in the common case (run PyFile_Check() and make
calls to stdio functions if it is a true file).

Even if this patch is rejected, the modified test_marshal.py
should probably be kept (with suitable deletions), as it refactors
the individual tests into helper functions, thus avoiding a lot
of code duplication and potential future cut-n-paste errors.