Implement a linked list data structure in Python.

It should support the following operations. Each operation should have a O(1) worst-case time complexity.

• push_head should add an element to the start of the list. It should return something that can be passed to remove to remove that element in the future.
• pop_tail should remove an element from the end of the list.
• remove takes a handle from push_head, and removes that element from the list.

There are some tests in linked_list_test.py for your implementation - feel free to write more.