* Validate and fix temp releasing (GH-3708)
Backports 92147baf.

    * Fix a temp leak in the type init code.
    * Fix temp leaks in fused types initialisation code.
    * Correctly release the buffer index temps allocated for index calculations.
    * Make tests fails hard if a temp variable is not released at the end of a generated function.
    * Fix temp leak in switch statement code.
    * Make end-to-end tests fail on refnanny output.
    * Fix result temp leak in PyTypeTestNode.
    * Fix result temp leak in external type/function import code and enable the refnanny check for them.
    * Fix temp leak when try-return-finally is used in generators.
    * Make it explicit when an allocated temp is not meant to be reused.
    * Fix temp leak when assigning to the real/imag attributes of complex numbers.
    * Fix temp leak when assigning to a memoryview slice.
    * Clean up "num_threads" result temp in parallel section, not only in prange loop.
    * Fix temp leak in Pythran buffer setitem code.
    * Simplify NumPyMethodCallNode since it does not need the Python function anymore. Previously, it generated code that needlessly looked up the Python function without actually using it.
    * Fix temp leak when deleting C++ objects.
    * Add a test that actually reusing temps when deleting C++ objects works correctly.

Fixes #3712 (hopefully) by reverting a small part of bbef4d74

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

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.

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.

Closes https://github.com/cython/cython/pull/3677

Allow selecting specific doctests in test modules with "-k pattern", instead of always running all tests.

Closes https://github.com/cython/cython/issues/3674

Return statements inside of nested try-except statements lead to an incorrect control flow graph that did not take the outer 'finally' clauses into account.
Closes https://github.com/cython/cython/issues/3666

classmethod() should fall back to calling PyClassMethod_New() instead of deciding for itself that the type is invalid. (GH-3660)

Fixes https://github.com/cython/cython/issues/3642

Fix detection of reusable utility code: whenever we do string and/or unbound method replacements at all, it's not reusable.