Rename an 's' to 'self'

This brings it in line with PEP8.
Remove setting of 'arena' as part of the GameObject class

It belongs in the instances, and is set in the constructor.
Move from one lock for all objects to one lock per object
Make the dynamic properties of game objects part of the instances rather than the class
Split the arena bounds properties into multiple lines

Again, in line with PEP8.
Move to 'self' rather than 's' for arena bounds properties
Remove an extraneous arena newline
Avoid all arena instances sharing the same objects list
Move to 'self' in arena.py
Pattern match (x, y) in the contains_point args
Change the loop variable to 'obj'

It's slightly more semantic than 'o'.
Factor wall marker population out into its own function
Further factor out the wall population
Remove an unused shebang line
Change 's' to 'self' in display.py
Change 'o' to 'obj' in display draw loop
Use 'is' for comparison with None
Reduce nesting in _draw
Use the 'not in' operator
Remove an extraneous newline