Port module to Stable ABI + multi-phase module initialization - #641

The `LDAP` type has been converted from a static type to a heap type.
The limited API does not support static types.

Heap types behave more closely like Python classes. They are allocated
on the heap and reference counted. Instances have a strong reference to
their type and must use GC protocol to track this reference.

The LDAP type can no longer be instantiated by Python code. This was
never supported and resulted in an invalid LDAP connection. Code like
`type(_ldap.initialize(""))()" now fails with a `TypeError`.

See: python-ldap#540
Signed-off-by: Christian Heimes <[email protected]>

The `_ldap` module now uses modern multi-phase module initialization.

Replace `LDAPadd_methods` hack with proper PyMethodDef for
module-level functions. The old approache is incompatible with
multi-phase init. Module-level functions are now prefixed with
`LDAPMod_` and exported.

Use `PyModuleDef_Slot` to initialize the `_ldap` C extension.

See: python-ldap#540
Signed-off-by: Christian Heimes <[email protected]>
Signed-off-by: Ondřej Kuzník <[email protected]>

Replace unsafe macros and direct struct access with functions from
the subset of limited API functions.

* `PySequence_Fast_GET_ITEM` -> `PySequence_GetItem`
* `PyTuple_SET_ITEM` -> `PyTuple_SetItem`
* `PyList_SET_ITEM` -> `PyList_SetItem`
* `const char *tp_name` -> `Py_TYPE()` string representation

See: python-ldap#540
Signed-off-by: Christian Heimes <[email protected]>

Python 3.12+ headers trigger it unless built with limited API.