[INTERNAL](typings) Use typing.Optional instead of typing.Union[type,NoneType]

[kumaraditya303]

Python's typing module has a dedicated Optional type for nullable types but in the playwright docs, typing.Union is being used and can be confusing for the users.

Solution:
Replace all occurrences of typing.Union with typing.Optional. @mxschmitt @pavelfeldman

[pavelfeldman]

I think 3.9 did it for us.