PyMemoryViewObject has a "base" field which points to the originator of
the buffer. However, this field has become redundant now that the
Py_buffer struct has received an "obj" field which also points to the
originator of the buffer (this has been done as part of the fix to #3139).

Not removing "base" would make for a confusing and error-prone API.
However, removing it is complicated by the fact that "base" is sometimes
abused to contain a tuple (for PyBUF_SHADOW type buffers, which are
neither mentioned in the PEP nor used anywhere in py3k).