PEP 821: Support for unpacking TypedDicts in Callable type hints (python#4764)

Co-authored-by: Hugo van Kemenade <1324225+hugovk@users.noreply.github.com>

Author: Daraan

.github/CODEOWNERS

@@ -697,6 +697,7 @@ peps/pep-0817/     @warsaw @dstufft
 # ...
 peps/pep-0819.rst  @emmatyping
 peps/pep-0820.rst  @encukou
+peps/pep-0821.rst  @JelleZijlstra
 peps/pep-0822.rst  @methane
 # ...
 peps/pep-2026.rst  @hugovk