Import issue with python <= 3.7

[elliotschrider]

When using python with version <= 3.7 there is an error due to this import on line 5:
from typing import Literal
Changing this to from typing_extensions import Literal for python <= 3.7 allows me to use optuna dashboard perfectly.

An easy fix that works for all python versions is:

try:
    from typing import Literal
except ImportError:
    from typing_extensions import Literal

Would it possible to implement this fix? Thanks.

[c-bata]

Thank you for reporting the bug. Yes, you are absolutely correct. Could you open a pull request if you have time?

[c-bata]

I've just opened #254 👍

[elliotschrider]

Forgot to write a thank you for this! Sorry that I was unable to submit the pull request before you, and thanks for taking the time to do it!

[c-bata]

No problem! Thank you for using optuna-dashboard 👍