python-chess is a pure Python chess library with move generation, move validation and support for common formats.
PyChess is also a pure Python, fairly mature, and will certainly be able to beat a casual player.
It's been a while since I've used PyChess, but a quick glance through some of the source does indicate that you can set a time limit on how long to search for a move.
The PyChess engine that is written in pure Python is in pychess.Utils. Specifically, if you look at pychess.Utils.lutils, you can see for instance the move generator written in Python.