BUG: Match exception declarations in scipy/io/matlab/streams.pyx to their corresponding pxd declarations.

In a recent [commit to Cython](https://github.com/cython/cython/commit/e6874284a11fb9cb7d5d5fb7b00c6451812aed20) stricter checking was added for function signatures declared in pyx and pxd files. This checking is now included in cython 21.1. This patch allows scipy to compile with the stricter checks.