Replace Class level variables with metaclass implementations

### What is wrong?
This snippet of code here is a `class level variable` and it would be better to define it using a `metaclass`.

https://github.com/ethereum/py_ecc/blob/b83b807e6ee058631e1904936c9795f913dfe342/py_ecc/fields/field_elements.py#L197-L201

### How can it be fixed
https://github.com/ethereum/py_ecc/pull/61#discussion_r263508419