Restore RDS DB from snapshot

[tewfik-ghariani]

This PR introduces the possibility of restoring an RDS DB from snapshot. Given that the RDS DB instance restored is automatically placed in the default SG, added the support for immediately modifying it to apply the submitted VPC SG

Additionally, the RDS module has been converted to boto3 instead of boto2

Other changes

• RDS Subnet group: fix the destroy operation
• Example file of RDS DB in VPC
• Blackify
• Upgrading boto3 version and re-locking poetry

cc @grahamc, @adisbladis, @RaitoBezarius