exec() did not allow recent Python syntax features in Py3.8+ due to https://bugs.python.org/issue35975
Closes https://github.com/cython/cython/issues/3695

Closes #1603.

Rebased 1bb26b96 for cython 0.29.x,
and made conditional to Python 3.  This does not solve the original staticmethod problem
in Python 2 but it does resolve classmethod problems in Python 3.9.  Therefore, it unbreaks
other packages on Python 3.9, improves overall state for Python 3 and does not introduce
regressions in Python 2.

This is a backwards incompatible change that enables Python semantics for special methods by default, while the directive allows users to opt out of it and go back to the previous C-ish Cython semantics.

See https://github.com/cython/cython/issues/2056

Re-add "c_api_binop_methods" directive for backwards compatibility after reverting https://github.com/cython/cython/pull/3633 and force it to "False".
Closes #3688.

This reverts commit e6a81240.

This reverts commit d849fb23.

This reverts commit bcb93877.

This reverts commit 02bb311d.

This reverts commit 66773260.