DOC: update 1.7.0-notes.rst from release branch

There were some changes in the release branch that were not put into master.
After this commit, the file 1.7.0-notes.rst is synchronised with the release
branch.
ENH: Use tp_basicsize, not NPY_SIZEOF_PYARRAYOBJECT, for memory allocation.

This change allows C-API level subtypes of PyArray_Type without forcing the
developer to rewrite the tp_alloc function and only change the amount of
memory allocated.
BUG: PyArray_LexSort allocates too much temporary memory.

PyArray_LexSort was allocating memory to hold actual PyArrayObject's
and PyArrayIterObject's, but only storing pointers to such objects
in the array.
Merge pull request #2976 from bfroehle/memory_waste

BUG: PyArray_LexSort allocates too much temporary memory.
Merge pull request #2975 from bfroehle/tp_alloc

ENH: Use tp_basicsize, not NPY_SIZEOF_PYARRAYOBJECT, for allocation
BUG: Make nansum work with booleans.

This broke when function_base._nannop tried to fill a boolean array with
integer zeros, raising a 'safe_casting' error. It looks like nanargmax and
nanargmin would also break, and were probably incorrect for booleans in any
case. The fix is not to use fill values for boolean and integer dtypes.
Previously that was only done for the integer dtypes.
Merge pull request #2979 from charris/fix-nansum

BUG: Make nansum work with booleans.