Use cx_Oracle.FIXED_NCHAR for sqltypes.NCHAR

Use cx_Oracle.FIXED_NCHAR for sqltypes.NCHAR

The :class:`.sqltypes.NCHAR` datatype will now bind to the
``cx_Oracle.FIXED_NCHAR`` DBAPI data bindings when used in a bound
parameter, which supplies proper comparison behavior against a
variable-length string.  Previously, the :class:`.sqltypes.NCHAR` datatype
would bind to ``cx_oracle.NCHAR`` which is not fixed length; the
:class:`.sqltypes.CHAR` datatype already binds to ``cx_Oracle.FIXED_CHAR``
so it is now consistent that :class:`.sqltypes.NCHAR` binds to
``cx_Oracle.FIXED_NCHAR``.

Fixes: #4913
Change-Id: I5bb111f2e06bbdd525bc5f716579baad31bbb3db