[issue17159] Remove explicit type check from inspect.Signature.from_function()

[Yury Selivanov]

Yury Selivanov added the comment:

> This change is redundant since BuiltinFunctionType (which isbuiltin() tests
> for) is already in _NonUserDefinedCallables, which is tested for right
> afterwards.

The code is more understandable, though.
Anyways, I'm glad that the issue is now resolved.

Also, please let's stop posting anything unrelated in this issue.

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue17159>
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