MAINT: random: Match type of SeedSequence.pool_size to DEFAULT_POOL_SIZE.

[charris]

Backport of #14303.

Fixes this compiler warning:

gcc: numpy/random/bit_generator.c
numpy/random/bit_generator.c:4889:41: warning: comparison of integers of different signs: 'int' and 'uint32_t' (aka 'unsigned int') [-Wsign-compare]
__pyx_t_6 = ((__pyx_v_self->pool_size != __pyx_v_5numpy_6random_13bit_generator_DEFAULT_POOL_SIZE) != 0);

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[charris]

The shippable error looks spurious, close/reopen to rerun.