gh-99845: Change _PyDict_KeysSize() return type to size_t (#99848)

gh-99845: Change _PyDict_KeysSize() return type to size_t (#99848)

* Change _PyDict_KeysSize() and shared_keys_usable_size() return type
  from signed (Py_ssize_t) to unsigned (size_t) type.
* new_values() argument type is now unsigned (size_t).
* init_inline_values() now uses size_t rather than int for the 'i'
  iterator variable.
* type.__sizeof__() implementation now uses unsigned (size_t) type.