2to3: Skip buffer fixer.

The buffer object is replaced by memoryview in Python >= 3. The memoryview object has also been backported to Python 2.7. However, the only use of buffer / memoryview is in `numpy/core/tests/test_unicode.py`
and there it is already version dependent: memoryview is used if the
Python version is >= 3 and buffer is used otherwise.

Closes #3043.