Treat s3:// urls as remote as htslib 1.3 onwards supports it

Created by: ah19

I've only added a super basic test for the method I changed rather than for the underlying s3 behaviour, I wasn't sure if you'd want a test going out to s3 as it might be flaky.

I've manually verified against htslib 1.3.2 that this does retrieve reads (via get_features_by_location) from a bam file in s3