Add a Valgrind build on debug Python 3.9 (#2746)

* Adding a valgrind build on debug Python 3.9

Co-authored-by: Boris Staletic <boris.staletic@gmail.com>

* Add Valgrind suppression files

- Introduce suppression file, populate it with a first suppression taken from CPython, and fix one leak in the tests
- Suppress leak in NumPy
- More clean tests!
- Tests with names a-e passing (except for test_buffer)
- Suppress multiprocessing errors
- Merge multiprocessing suppressions into other suppression files
- Numpy seems to be spelled with a big P
- Append single entry from valgrind-misc.supp to valgrind-python.supp, and make clear valgrind-python.supp is only CPython

Co-authored-by: Boris Staletic <boris.staletic@gmail.com>

* Enable test_virtual_functions with a workaround

* Add a memcheck cmake target

- Add a memcheck cmake target
- Reformat cmake
- Appease the formatting overlords - they are angry
- Format CMake valgrind target decently

* Update CI config to new action versions

* fix: separate memcheck from pytest

* ci: cleanup

* Merge Valgrind and other deadsnakes builds

Co-authored-by: Boris Staletic <boris.staletic@gmail.com>
Co-authored-by: Henry Schreiner <henryschreineriii@gmail.com>

tests/test_virtual_functions.py

@@ -251,8 +251,7 @@ def test_dispatch_issue(msg):
                 == 'Tried to call pure virtual function "Base::dispatch"'
             )
 
-            p = PyClass1()
-            return m.dispatch_issue_go(p)
+            return m.dispatch_issue_go(PyClass1())
 
     b = PyClass2()
     assert m.dispatch_issue_go(b) == "Yay.."