Fix compiler warning

Author: 1st1

codecs/text.pyx

@@ -12,7 +12,7 @@ cdef inline as_pg_string_and_size(
         raise TypeError('expected str, got {}'.format(type(obj).__name__))
 
     if settings.is_encoding_utf8():
-        cstr[0] = cpythonx.PyUnicode_AsUTF8AndSize(obj, size)
+        cstr[0] = <char*>cpythonx.PyUnicode_AsUTF8AndSize(obj, size)
     else:
         encoded = settings.get_text_codec().encode(obj)
         cpython.PyBytes_AsStringAndSize(encoded, cstr, size)

cpythonx.pxd

@@ -13,4 +13,5 @@ cdef extern from "Python.h":
     char* PyByteArray_AsString(object)
 
     object PyUnicode_FromString(const char *u)
-    char* PyUnicode_AsUTF8AndSize(object unicode, ssize_t *size) except NULL
+    const char* PyUnicode_AsUTF8AndSize(
+        object unicode, ssize_t *size) except NULL