[3.12] gh-116767: fix crash on 'async with' with many context managers (GH-118348...

[3.12] gh-116767: fix crash on 'async with' with many context managers (GH-118348) (#118477)

gh-116767: fix crash on 'async with' with many context managers (GH-118348)

Account for `add_stopiteration_handler` pushing a block for `async with`.
To allow generator functions that previously almost hit the `CO_MAXBLOCKS`
limit by nesting non-async blocks, the limit is increased by 1.
This increase allows one more block in non-generator functions.

(cherry picked from commit c1bf4874c1e9db2beda1d62c8c241229783c789b)