The answer above by @LoicTheAztec works perfectly and is very clear. I'm adding this answer because I'm not sure it's correct to put additional suggestions and code in a comment (sorry if I'm not following proper StackOverflow protocols -- someone please let me know if that's the case!).
Anyway even after changing the password strength requirement I was still seeing the very stern and rather unhelpful password hint demanding twelve characters &c., so I went looking for a way to change that. Here are the two functions I've got running and they're working just as expected.
For the password hint function, thanks to arjenlentz.
// First, change the required password strength
add_filter( 'woocommerce_min_password_strength', 'reduce_min_strength_password_requirement' );
function reduce_min_strength_password_requirement( $strength ) {
// 3 => Strong (default) | 2 => Medium | 1 => Weak | 0 => Very Weak (anything).
return 2;
}
// Second, change the wording of the password hint.
add_filter( 'password_hint', 'smarter_password_hint' );
function smarter_password_hint ( $hint ) {
$hint = 'Hint: longer is stronger, and consider using a sequence of random words (ideally non-English).';
return $hint;
}