bpo-39937: modify iter() suggestion to Element.iter()

[kumiDa]

In the whatsnew section, under the point which mentions the deprecation of getchildren() and getiterator()
through bpo-36543, it is suggested to use iter() instead.

Ideally there should be a suggestion to use Element.iter() instead.

The commits does the above modification.

https://bugs.python.org/issue39937

[the-knights-who-say-ni]

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[tirkarthi]

NEWS entry is not needed. Please remove it.

[tirkarthi]

LGTM. Thanks.

[tirkarthi]

@serhiy-storchaka It will be helpful to have your review in this since it caused some confusion over builtin function iter to be used at biopython/biopython#2774 (comment)

[tirkarthi]

This change is for 3.9 in whatsnew. I think backport to 3.8 is not needed.

[tirkarthi]

Thanks @serhiy-storchaka for the review. Congratulations @rahul-kumi on your first patch :)

[kumiDa]

Thanks @serhiy-storchaka for the review. Congratulations @rahul-kumi on your first patch :)

Thank you @tirkarthi and @serhiy-storchaka !