Add missing Cython headers for the new PyUnicode API

pyfastani/_unicode.pxd

+from libc.stdint cimport uint8_t, uint16_t, uint32_t
+
+
+cdef extern from *:
+
+    ctypedef uint8_t  Py_UCS1
+    ctypedef uint16_t Py_UCS2
+    ctypedef uint32_t Py_UCS4
+
+    object PyUnicode_New(Py_ssize_t, Py_UCS4)
+    int PyUnicode_WriteChar(object, Py_ssize_t, Py_UCS4) except -1
+    Py_UCS4 PyUnicode_ReadChar(object, Py_ssize_t)
+
+    cdef int PyUnicode_1BYTE_KIND
+    cdef int PyUnicode_2BYTE_KIND
+    cdef int PyUnicode_4BYTE_KIND
+
+    ssize_t PyUnicode_GET_LENGTH(object)
+    int     PyUnicode_KIND(object)
+    void*   PyUnicode_DATA(object)
+    void    PyUnicode_WRITE(int kind, void *data, Py_ssize_t index, Py_UCS4 value) nogil
+    Py_UCS4 PyUnicode_READ (int kind, void *data, Py_ssize_t index) nogil
+
+    Py_UCS1* PyUnicode_1BYTE_DATA(object) nogil
+    Py_UCS2* PyUnicode_2BYTE_DATA(object) nogil
+    Py_UCS4* PyUnicode_4BYTE_DATA(object) nogil
+
+    object PyUnicode_FromStringAndSize(const char*, Py_ssize_t)
+    object PyUnicode_FromKindAndData(int kind, const void *buffer, Py_ssize_t size)
+
+    int    PyUnicode_READY(object) except -1