getpass.getuser() raises OSError in Python 3.13

https://github.com/python/cpython/commit/99a73c3465a45fe57cac01a917fc50e0743b5964#diff-3bde66c820a115e5740d481232a0c48331c3856ad4c247a586be98991c08f8cbR157

`getpass.getuser()` raises `OSError` in Python 3.13. [The code that gets `DEFAULT_USER`](https://github.com/PyMySQL/PyMySQL/blob/ec27bade879ad05fda214188d035c1fe3f255a35/pymysql/connections.py#L43) must catch `OSError` now.