silx.gui.plot: Added Python typing

[t20100]

Merge PR #3956 first!

This PR adds some Python typing on existing functions/methods done while working on PR #3956.
To make it easier to review I splitted it in 2 PRs.
This one only containing the typing once PR #3956 is merged

[vallsv]

You can replace

• typing.Any by object
• typing.Union[a, b] by a | b
• typing.Optional[a] by a | None

That's less stuffs to import

But this enforce to import the future annotation.

[t20100]

I didn't knew you can use | with types with Python3.7, that's great: https://peps.python.org/pep-0604/#change-only-pep-484-type-hints-to-accept-the-syntax-type1-type2

[t20100]

Updated typing.Union[a, b] -> a | b and typing.Optional[a] -> a | None