Fix config(exclude=…) type annotations

[andersk]

• The exclude callback takes one argument, not two.
• Objects of type Exclude are not accepted. ALWAYS and NEVER have type Callable, not Exclude.
• Class variables like Exclude.ALWAYS, Exclude.NEVER are not allowed to be generic, and they don’t need to be, since Callable[[object], bool] is convertible to Callable[[T], bool].

[andersk]

Apparently I’m not the first to submit this.

• #231
• #336
• #360

[george-zubrienko]

hi @andersk could you please update your PR so we can review and merge :)

[andersk]

@george-zubrienko Rebased.