fix: valgrind-detected after-freeing access of PyMethodDef (macOS Python 3.9.0 segfaults)

[YannickJadoul]

Python 3.9 added this line to meth_dealloc, unfortunately for us, 2 lines too low.

Short summary of what goes wrong:

• pybind11 stores the function_record in a capsule, and passes this as the self argument to PyCFunction_NewEx
• The PyCFunctionObject object nicely keeps track of that, but decrefs m->m_self 2 lines before decrefing PyCFunction_GET_CLASS(m). The capsule with the function_record with the PyModuleDef gets freed.
• PyCFunction_GET_CLASS accesses m_ml -> ml_flags, where m_ml now points to something that was destructed.
• BOOM

I guess this being so close explains why we don't often get a crash here. Maybe macOS protects its memory better?

• 61e40f6 demonstrates that leaking the PyModuleDef resolves the Python 3.9 crash on macOS.
• d10f154 would be a (the?) solution, if it weren't calling cpp_function during the construction of a cpp_function during the construction of a cpp_function during the construction of a cpp_function during ... and thus blowing up the stack.

Closes #2558

Related issue on bugs.python.org: https://bugs.python.org/issue41237
PR on CPython adding the line to meth_dealloc that makes things crash: python/cpython#19936

[YannickJadoul]

Replacing delete rec->def; with

                static int counter = 0;
                std::cout << ++counter << std::endl;

and running pybind11's tests, gives me a final count of 1808, of which 49 before cleaning up (so things that would be cleaned up during, by pybind11, that are the true leaks, here):

======================================== test session starts ========================================
platform linux -- Python 3.8.6, pytest-5.3.2, py-1.8.0, pluggy-0.13.1
rootdir: /home/yannick/pybind11/tests, inifile: pytest.ini
plugins: lazy-fixture-0.6.2
collected 430 items                                                                                 

../../tests/test_async.py ..                                                                  [  0%]
../../tests/test_buffers.py .........                                                         [  2%]
../../tests/test_builtin_casters.py ....s...........                                          [  6%]
../../tests/test_call_policies.py ....1
.2
...                                                    [  8%]
../../tests/test_callbacks.py 3
4
...5
.6
.....                                                       [ 10%]
../../tests/test_chrono.py ...........................................                        [ 20%]
../../tests/test_class.py .............7
8
......9
10
.........11
12
13
.                                       [ 26%]
../../tests/test_constants_and_functions.py ....                                              [ 27%]
../../tests/test_copy_move.py ..14
15
16
17
..s..                                                         [ 29%]
../../tests/test_custom_type_casters.py ..                                                    [ 30%]
../../tests/test_docstring_options.py .                                                       [ 30%]
../../tests/test_eigen.py .........................                                           [ 36%]
../../tests/test_enum.py ......                                                               [ 37%]
../../tests/test_eval.py 18
19
.20
.                                                                   [ 37%]
../../tests/test_exceptions.py .........                                                      [ 40%]
../../tests/test_factory_constructors.py ....21
.22
..23
........                                      [ 43%]
../../tests/test_gil_scoped.py 24
25
26
27
.....                                                          [ 44%]
../../tests/test_iostream.py ............                                                     [ 47%]
../../tests/test_kwargs_and_defaults.py ........                                              [ 49%]
../../tests/test_local_bindings.py ..........                                                 [ 51%]
../../tests/test_methods_and_attributes.py .....28
........29
.......                               [ 56%]
../../tests/test_modules.py .....                                                             [ 57%]
../../tests/test_multiple_inheritance.py ........30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
...                                          [ 60%]
../../tests/test_numpy_array.py ................................................              [ 71%]
../../tests/test_numpy_dtypes.py ...............                                              [ 74%]
../../tests/test_numpy_vectorize.py ........                                                  [ 76%]
../../tests/test_opaque_types.py ...                                                          [ 77%]
../../tests/test_operator_overloading.py ....                                                 [ 78%]
../../tests/test_pickling.py .....                                                            [ 79%]
../../tests/test_pytypes.py ...............................                                   [ 86%]
../../tests/test_sequences_and_iterators.py ........                                          [ 88%]
../../tests/test_smart_ptr.py .............                                                   [ 91%]
../../tests/test_stl.py ........ss........                                                    [ 95%]
../../tests/test_stl_binders.py .........                                                     [ 97%]
../../tests/test_tagbased_polymorphic.py .                                                    [ 97%]
../../tests/test_union.py .                                                                   [ 98%]
../../tests/test_virtual_functions.py .46
47
.48
.49
.....                                                [100%]

====================================== short test summary info ======================================
SKIPPED [1] test_builtin_casters.py:135: no <string_view>
SKIPPED [1] test_copy_move.py:69: no <optional>
SKIPPED [1] test_stl.py:109: no <optional>
SKIPPED [1] test_stl.py:137: no <experimental/optional>
================================== 426 passed, 4 skipped in 5.81s ===================================