Fix hough_circle regression on windows

This adds back the pure Python interface for `hough_circle` so that the radius argument can be cast to `np.intp_t`.

This issue was fixed in https://github.com/scikit-image/scikit-image/commit/7ca08cf7630787201636da4ea4409741b558d613 (which essentially has the same fix deployed here) and then broken in https://github.com/scikit-image/scikit-image/pull/480 (which I reviewed, apparently).