Avoid deprecated utcnow for AWS timestamps

[Mirochill]

Fixes #71.

Summary

• use datetime.datetime.now(datetime.timezone.utc) when creating AWS Signature V4 timestamps
• keep a fallback for older Python versions that do not expose datetime.timezone
• update the time mocks in the existing signing tests so the expected signatures remain fixed

Validation

• git diff --check
• Not run locally