bpo-36431: Use PEP 448 dict unpacking for merging two dicts.

[serhiy-storchaka]

https://bugs.python.org/issue36431

[tirkarthi]

Slightly confused over why allow duplicate comment is added here since dict merge removes duplicates though self.keywords and keyword might have same keys.

[serhiy-storchaka]

I wanted to mention the reason the reason of why we cannot just call self.func(*self.args, *args, **self.keywords, **keywords). The latter syntax does not allow an argument name be duplicated in self.keywords and keywords.

I'll clarify the comments.

[serhiy-storchaka]

I have just removed unclear comments. The behavior is explained in the module documentation and covered by tests, so there is small chance to break it.

Interesting that the reference implementation in the documentation already uses dict unpacking.

[terryjreedy]

I like all the changes, especially the last one in functools.
Please don't close the issue until the idlelib change is backported one way or another.

[ericvsmith]

All of these changes look correct to me.