[3.7] bpo-35035: Rename email.utils documentation to email.utils.rst (GH-10023)

I'll watch for 404 on the old URL and will setup an HTTP redirection if needed.
(cherry picked from commit 361e8683e7340c600b22f4a514b81448ccec66dc)

Co-authored-by: Zhiming Wang <github@zmwang.pw>
3 files changed