chore(deps): update dependency b2sdk to v1.21.0

This MR contains the following updates:

Package	Update	Change
b2sdk	minor	==1.19.0 -> ==1.21.0

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Release Notes

Backblaze/b2-sdk-python

v1.21.0

Compare Source

Added

• Add support for custom upload timestamp
• Add support for cache control header while uploading

Infrastructure

• Remove dependency from arrow
• Build Python wheels for distribution

v1.20.0

Compare Source

Added

• Add use_cache parameter to B2Api.list_buckets

Changed

• Connection timeout is now being set explicitly

Fixed

• Small files downloaded twice

Infrastructure

• Disable changelog verification for dependabot MRs

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