bpo-39037: Fix lookup order of magic methods in with statement documentation (GH...

bpo-39037: Fix lookup order of magic methods in with statement documentation (GH-17608)

* __enter__ is now looked up before __exit__ to give a more intuitive error message
* add pseudo-code equivalent for the with statement
* fix pseudo-code for the async with statement to use a finally clause
* use SUITE rather than BLOCK for consistency with the language grammar

Patch by Géry Ogam.
(cherry picked from commit 226e6e7d4326cf91ef37e13528eb1f62de1bb832)

Co-authored-by: Géry Ogam <gery.ogam@gmail.com>