Tic Tac Toe Game in Python 3
First of all, I suggest you read pep8 (https://pep8.org/). I see you have violations. Understanding pep8 is a must if you want to be a pro Python developer. Better yet, use flake8 to automatically find pep8 violations in your code.

Second, using that many elif statements is always a red flag, especially when they seem to be doing the same thing. Can you somehow get rid of that duplication?

Next, put some blank lines between logical blocks of code to improve readability. In Game.start the try catch block is logically separate from the conditional following it, for example.

Next, give your variables meaningful names. For example, game.start is much better than obj.start. Also, avoid using shortened version of words, like object -> obj because it doesn't add much value and can decrease readability.

Finally, I'd like to see some tests. Not because I think your code has a bug but because I want to know how your code actually works. Because there's some logical reasoning in the nature of the problem, you can help the reader by documenting the algorithm you've implemented. Unit tests are perfect for this. They're fast, they target specific conditions one at a time, and document your code better than comments.
If you're wondering where to start when it comes to testing your code I suggest watching some videos on TDD in action. Kent Beck and Uncle Bob have some free videos showing how to do TDD step by step.