PEP 837: Extensible JSON serialization (GH-5031)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: serhiy-storchaka

.github/CODEOWNERS

@@ -711,6 +711,7 @@ peps/pep-0832.rst  @brettcannon
 peps/pep-0833.rst  @dstufft
 peps/pep-0835.rst  @ilevkivskyi
 peps/pep-0836.rst  @savannahostrowski @Fidget-Spinner @brandtbucher
+peps/pep-0837.rst  @serhiy-storchaka
 peps/pep-0838.rst  @AlexWaygood
 peps/pep-0840.rst  @jeremyhylton @gvanrossum
 # ...