Deprecate Python 3.6

Summary

Python 3.6 has reached end of life last year on December 23rd, 2021.

Proposed benefits

Python 3.6 compatibility can be removed throughout the codebase.

Proposal details

• Remove 3.6 from CI matrix
• Remove dependency constraints for Python 3.6
• Remove any compatibility hacks for 3.6 (e.g. those in singer_sdk/helpers/_compat.py)
• Update any "old" Python syntax to >= 3.7 (via pyupgrade)

Best reasons not to build

It's really not that bad to continue supporting it for a while.