more type annotation fixes

[dimbleby]

simplifying and - I think! - correcting more python type annotations. Not so straightforward as #4121, you'll probably want to think about it...

• an IntVar is a LinearExpr, so a union containing them both is pointless, it might as well just contain a LinearExpr
• likewise an Integral is already a Number

then try to correct the annotations for __add__ etc, by making them all just return a new LinearExpr - which I am fairly sure is true.

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