I write with such hesitation, knowing that I will inspire the wrath and disapprobation of some rabid descriptivist by presuming to think such thoughts as these about the English language. But, insel, you asked.
"None" is an indefinite pronoun. A singular pronoun takes a singular verb. A plural pronoun takes a plural verb. None may be thought of as either singular or plural, wherein lies the rub. If one thinks of "none" as the two-word phrase "not one", one will use a singular verb. If one thinks of "none" as the two-word phrase "not any", one will use a plural verb.
None (not one) of the students is going to the assembly.
None (not any) of the students are going to the assembly.
None of the fruit is ripe.
None (not any) of the fruits are ripe.
None (not one) of the fruits is ripe.
Of course, if it doesn't matter to you, then it doesn't matter.
