Upgrade default version of Python from 3.6

[swcurran]

We need to update from Python 3.6, and so need a plan to do so. PR #1854 has been created to start the process.

We discussed this at the ACA-Pug meeting on July 12, notes copied below.

• 3.6 is end of life'd, so this is a high priority
• We have a 3.7 PR, but 3.7 is less than a year from end of life
• Other versions lifespans
  • Why not use 3.10?
• Meeting Discussion:
  • On 3.7 branch, all tests are passing, but with lots of deprecation warnings
  • 3.6 to 3.7 is likely the biggest breaking change (vs. going to 3.8, 3.9, etc.)
    • 3.11 evidently has a significant performance improvement (e.g. 25%), so being ready is important
  • Suggested approach:
    • 3.7 branch for now
    • Add GHAs to run all tests on all Python versions 3.7 - 3.10.
      • Run one version on PRs (same as today)
      • Run other versions daily on a cron
    • Add tag-based publishing of artifacts, and publish to the Hyperledger GitHub Container Repository
    • @burdettadam and team to produce a HackMD doc about the plans to promote comment and community collaboration