[3.8] bpo-3832: Fix compiler warnings (GH-16518)

[3.8] bpo-3832: Fix compiler warnings (GH-16518)

* bpo-38321: Fix _asynciomodule.c compiler warning (GH-16493)

bpo-38248, bpo-38321: Fix warning:

    modules\_asynciomodule.c(2667):
    warning C4102: 'set_exception': unreferenced label

The related goto has been removed by
commit edad4d89e357c92f70c0324b937845d652b20afd.

(cherry picked from commit efe74b6369a8d08f27c69703fcc1686966e51068)

* bpo-38321: Fix PyCStructUnionType_update_stgdict() warning (GH-16492)

bpo-22273, bpo-38321: Fix following warning:

    modules\_ctypes\stgdict.c(704):
    warning C4244: 'initializing': conversion from 'Py_ssize_t' to 'int', possible loss of data

(cherry picked from commit c9a413ede47171a224c72dd34122005170caaad4)