Move NaT to self-contained module (#18014)

Author: jbrockmendel

pandas/_libs/period.pyx

@@ -30,10 +30,11 @@ from pandas._libs import tslib
 from pandas._libs.tslib import Timestamp, iNaT, NaT
 from tslibs.timezones cimport (
     is_utc, is_tzlocal, get_utcoffset, get_dst_info, maybe_get_tz)
-from tslib cimport _nat_scalar_rules
 
 from tslibs.parsing import parse_time_string, NAT_SENTINEL
 from tslibs.frequencies cimport get_freq_code
+from tslibs.nattype import nat_strings
+from tslibs.nattype cimport _nat_scalar_rules
 
 from pandas.tseries import offsets
 from pandas.tseries import frequencies
@@ -1174,7 +1175,7 @@ class Period(_Period):
                 converted = other.asfreq(freq)
                 ordinal = converted.ordinal
 
-        elif is_null_datetimelike(value) or value in tslib._nat_strings:
+        elif is_null_datetimelike(value) or value in nat_strings:
             ordinal = iNaT
 
         elif is_string_object(value) or util.is_integer_object(value):

pandas/_libs/src/inference.pyx

@@ -2,7 +2,7 @@ import sys
 from decimal import Decimal
 cimport util
 cimport cython
-from tslib import NaT
+from tslibs.nattype import NaT
 from tslib cimport convert_to_tsobject, convert_to_timedelta64
 from tslibs.timezones cimport get_timezone
 from datetime import datetime, timedelta

pandas/_libs/src/ujson/python/objToJSON.c

@@ -162,7 +162,7 @@ void initObjToJSON(void)
 #endif
 {
     PyObject *mod_pandas;
-    PyObject *mod_tslib;
+    PyObject *mod_nattype;
     PyObject *mod_decimal = PyImport_ImportModule("decimal");
     type_decimal = PyObject_GetAttrString(mod_decimal, "Decimal");
     Py_INCREF(type_decimal);
@@ -180,10 +180,11 @@ void initObjToJSON(void)
         Py_DECREF(mod_pandas);
     }
 
-    mod_tslib = PyImport_ImportModule("pandas._libs.tslib");
-    if (mod_tslib) {
-        cls_nat = (PyTypeObject *)PyObject_GetAttrString(mod_tslib, "NaTType");
-        Py_DECREF(mod_tslib);
+    mod_nattype = PyImport_ImportModule("pandas._libs.tslibs.nattype");
+    if (mod_nattype) {
+        cls_nat = (PyTypeObject *)PyObject_GetAttrString(mod_nattype,
+                                                         "NaTType");
+        Py_DECREF(mod_nattype);
     }
 
     /* Initialise numpy API and use 2/3 compatible return */

pandas/_libs/tslib.pxd

@@ -2,7 +2,6 @@ from numpy cimport ndarray, int64_t
 
 cdef convert_to_tsobject(object, object, object, bint, bint)
 cpdef convert_to_timedelta64(object, object)
-cdef bint _nat_scalar_rules[6]
 cdef bint _check_all_nulls(obj)
 
 cdef _to_i8(object val)