API Fixes: Python client: Add tests for buffer.valid
API Fixes: Python client: Add tests for buffer.append
API Fixes: Fix buffer_get_mark
API Fixes: Python client: Add tests for buffer.mark
API Fixes: Python client: Add tests for window.buffer
API Fixes: Python client: Add tests for window.cursor
API Fixes: Python client: Add tests for window.height
API Fixes: Python client: Add tests for window.width
API Fixes: Python client: Add tests for window.vars
API Fixes: Python client: Add tests for window.options
API Fixes: Python client: Add tests for window.row/col
API Fixes: Python client: Add tests for window.valid/tabpage