PEP 590: Use size_t for "number of arguments + flag" (GH-1066)

Bitwise operations are not well defined on signed values in C.

pep-0590.rst

@@ -65,11 +65,11 @@ Calls are made through a function pointer taking the following parameters:
 
 * ``PyObject *callable``: The called object
 * ``PyObject *const *args``: A vector of arguments
-* ``Py_ssize_t nargs``: The number of arguments plus the optional flag ``PY_VECTORCALL_ARGUMENTS_OFFSET`` (see below)
+* ``size_t nargs``: The number of arguments plus the optional flag ``PY_VECTORCALL_ARGUMENTS_OFFSET`` (see below)
 * ``PyObject *kwnames``: Either ``NULL`` or a tuple with the names of the keyword arguments
 
 This is implemented by the function pointer type:
-``typedef PyObject *(*vectorcallfunc)(PyObject *callable, PyObject *const *args, Py_ssize_t nargs, PyObject *kwnames);``
+``typedef PyObject *(*vectorcallfunc)(PyObject *callable, PyObject *const *args, size_t nargs, PyObject *kwnames);``
 
 Changes to the ``PyTypeObject`` struct
 --------------------------------------
@@ -138,7 +138,7 @@ New C API and changes to CPython
 
 The following functions or macros are added to the C API:
 
-- ``PyObject *_PyObject_Vectorcall(PyObject *obj, PyObject *const *args, Py_ssize_t nargs, PyObject *keywords)``:
+- ``PyObject *_PyObject_Vectorcall(PyObject *obj, PyObject *const *args, size_t nargs, PyObject *keywords)``:
   Calls ``obj`` with the given arguments.
   Note that ``nargs`` may include the flag ``PY_VECTORCALL_ARGUMENTS_OFFSET``.
   The actual number of positional arguments is given by ``PyVectorcall_NARGS(nargs)``.
@@ -152,7 +152,7 @@ The following functions or macros are added to the C API:
   ``*args`` and ``**kwargs`` calling convention.
   This is mostly meant to put in the ``tp_call`` slot.
 
-- ``Py_ssize_t PyVectorcall_NARGS(Py_ssize nargs)``: Given a vectorcall ``nargs`` argument,
+- ``Py_ssize_t PyVectorcall_NARGS(size_t nargs)``: Given a vectorcall ``nargs`` argument,
   return the actual number of arguments.
   Currently equivalent to ``nargs & ~PY_VECTORCALL_ARGUMENTS_OFFSET``.