PEP 728: TypedDict with Typed Extra Fields (#3441)

Signed-off-by: Zixuan James Li <[email protected]>
Co-authored-by: Jelle Zijlstra <[email protected]>

Author: PIG208

.github/CODEOWNERS

@@ -607,6 +607,7 @@ peps/pep-0724.rst  @jellezijlstra
 peps/pep-0725.rst  @pradyunsg
 peps/pep-0726.rst  @AA-Turner
 peps/pep-0727.rst  @JelleZijlstra
+peps/pep-0728.rst  @JelleZijlstra
 peps/pep-0729.rst  @JelleZijlstra @hauntsaninja
 peps/pep-0730.rst  @ned-deily
 peps/pep-0731.rst  @gvanrossum @encukou @vstinner @zooba @iritkatriel