"If you want to announce that Python 2.7.x supports large objects on Windows 64 bits, be prepared to have to fix Python in various different places."

You can compare which modules define PY_SSIZE_T_CLEAN in Python 2.7 and 3.x. For example, it looks like bz2 and zlib modules handle correctly 64-bit lengths in Python 3, but don't in Python 2.

By the way, BZ2File_read() in Python 2.7 uses also the "l" format to parse the input length. It looks like the code was copied from fileobject.c.