gh-85453: Adapt datetime.rst to devguide recommendations for code snippets and variables (#118068)

Also remove formatting from numeric literals.

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>
Co-authored-by: Erlend E. Aasland <erlend@python.org>