Drop python2 support

[danigm]

Python 2 went EOL on 1/1/2020 and we receive no more support from the Python Core Team after Python 2.7.18 even for known security issues.

[avylove]

You can see our discussion on this when it came up before in #170, but here are the bullet points

• People are still using 2.7
  • Blessed has had over 200k downloads for 2.7 in the last 6 months
• Third parties, like Red Hat, are still maintaining 2.7
  • RHEL 7 is scheduled to go EOL June 2024. I think that's when we should be looking at dropping it
• There are downstream effects
  • Without a good reason, we don't want to put a burden on other developers
• It's not a huge gain for us
  • There are only a few places this would result in code optimizations and they are minor
  • The maintenance burden isn't very high

We dropped 2.6 about 1.5 years ago because usage was very low and it was a burden to maintain and test. I expect 3.4 will be next since there were only 1.6k downloads in the last 6 months and we're only testing it in CI with the OS image pinned to an old version. 3.5 probably isn't far behind because downloads were only 25k and it's slowly disappearing from repos. 3.6 will likely be around for a long time because a lot of software standardized on it.