[3.9] bpo-25130: Make unit-test about restricting the maximum number of nested blocks cpython-only (GH-28002) (GH-28017)

PyPy and potentially other implementations have different or no
contraints on the number of blocks that can be statically nested. move
the test that checks for this behaviour into a unit test and mark it as
CPython-only..
(cherry picked from commit eb263f9a356f5c5f21b8d5ce20bac92f31c40cad)

Co-authored-by: Carl Friedrich Bolz-Tereick <cfbolz@gmx.de>