Fix reading empty file or seeking past end of file for s3 backend

[jcushman]

Motivation

This PR fixes seeking past the end of an s3 file, which I broke in #495. E.g. this will now work instead of raising a boto error:

with open('s3://some/random/uri', 'rb') as file:
    file.seek(1_000_000_000)

file.tell() will then give the current position as the actual end of the file, and f.read() will return b''.

This also fixes opening an empty file for reading, as reported in #548 -- that turns out to be a special case of seeking past the end of a file being broken.

The reason this wasn't working before is that moto doesn't return the same API response as real S3 when requesting an invalid range with GetObject, as reported here. We were correctly handling moto-style responses but failing to handle real-S3-style responses. So this PR:

• Updates _SeekableRawReader to expect responses as provided by real S3
• Mocks _get in the relevant tests to return real-S3-style responses. This mock could be removed when the upstream bug in moto is fixed
• Adds tests for reading an empty file and seeking past the end of a file

Fixes #548

Checklist

Before you create the PR, please make sure you have:

• Picked a concise, informative and complete title
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• Checked that all unit tests pass

[mpenkov]

LGTM!

[jcushman]

FYI that the moto mock for testing will apparently be unnecessary after the next release of moto -- getmoto/moto#2981

[mpenkov]

Merged. Thank you for your contribution and your patience @jcushman !

[Andrew-Sheridan]

@mpenkov I'm curious, when is the next release is scheduled?

Would be nice to have this updated logic available.

[mpenkov]

I expect to release within a week from now.