The 'DEF' statement is deprecated

[jwilk]

When building with Cython 3.0.0b1, I get a few warnings like this:

afl.pyx:40:0: The 'DEF' statement is deprecated and will be removed in a future Cython version. Consider using global variables, constants, and in-place literals instead. See https://github.com/cython/cython/issues/4310

These are the affected lines:

DEF SHM_ENV_VAR = b'__AFL_SHM_ID'
DEF FORKSRV_FD = 198
DEF MAP_SIZE_POW2 = 16
DEF MAP_SIZE = 1 << MAP_SIZE_POW2