BUG: fix uninitialized use in _strided_to_strided_string_to_datetime

dt == NPY_DATETIME_NAT is used as error condition, so initialize it to
something else.
BUG: fix NaT handling in _strided_to_strided_string_to_datetime

When converting an array of strings to datetime64, a "NaT" occurring
in the input would cause all subsequent values to be converted to NaT.
BUG: Fix missing NULL check in lexsort

When Getitem of the lexsort sequence argument failed
this was not checked.

Closes gh-4123
BUG: fix missing check for memory allocation failure in ufuncs

closes gh-4018
MAINT: don't use open(.., 'U') in Python 3.x, is deprecated.

This is default behavior in 3.x; in 2.x open() doesn't have a `newline`
kw.  So make code Python-version-specific.
TST: fix test failure due to comparing floats with assert_equal.

Observed on 32-bit linux with python 3.4b1
DOC: update release notes with new fixes since rc1
Merge pull request #4157 from juliantaylor/backport-fixes

Backport fixes