1) isfunction() returns `True` for partial object only when one passes `func` attribute of it.
2) For instance, `isgeneratorfunction()` and `iscoroutinefunction()` for partial obj work with passing partial obj as well as with passing `func` attr of this obj, when obj is partially applied generator function or partially applied coroutine function respectively.

I offer to unify behavior for handling partial object for r'inspect.is*function()' by the next way:
1) Add `functools._unwrap_partial()` to `inspect.isfunction()` as well as it were done in other r'inspect.is*function()'.

P.S.I'm ready to deal with this issue.  

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