Python 3 two-sum performance
I think your solution is already pretty good. It is at worst \$\mathcal{O}(n)\$ in both runtime and memory. You use enumerate instead of iterating over the indicies manually.

The only four things I would comment on are:


In real life code, this being a class would of course  be completely unnecessary (but it seems to be mandatory in this case).
Explicitly returning None at the end. This is always a judgement call between being unnecessarily verbose (since it is returned implicitly anyways) and being more expressive. Since this case is explicitly excluded in the problem description (all inputs have exactly one solution), I would not add it here. If it were an explicit rule to return None in case there was no solution I would, however, include it.
You should add some docstring to the class/method. At least to get in the habit of always doing it.
Python's official style-guide, PEP8, recommends using lower_case for variable, function and method names (but this seems to also be mandatory from the problem description).