TST: update test_series_factorize_na_sentinel_none for 32bit
xref https://github.com/pandas-dev/pandas/issues/35831#issuecomment-688163069

(I've not tested this locally, after backport can retrigger https://github.com/MacPython/pandas-wheels/pull/97 to check)
thanks @simonjayhawkins  @meeseeksdev backport 1.1.x