bpo-1635741: Port _stat module to multiphase initialization

[corona10]

https://bugs.python.org/issue1635741

[corona10]

@vstinner Please take a look

[vstinner]

LGTM, thanks for the updates!

[vstinner]

The new code emits a compiler warning:

./Modules/_stat.c: In function ‘stat_exec’:
./Modules/_stat.c:566:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for (int i = 0; i < Py_ARRAY_LENGTH(st_constants); i++) {
                       ^

Py_ARRAY_LENGTH() should be casted to int.

cc @corona10

[vstinner]

Py_ARRAY_LENGTH() should be casted to int.

After I saw the warning 5 times in a row, I decided to propose a PR for the trivial fix :-)
#19822