Deprecate Python2.7.

[laixintao]

So we can use latest version of prompt-toolit.
Relate: #1149

Checklist

• I've added this contribution to the changelog.rst.
• I've added my name to the AUTHORS file (or it's already there).

• I installed pre-commit hooks (pip install pre-commit && pre-commit install), and ran black on my code.
• Please squash merge this pull request (uncheck if you'd like us to merge as multiple commits)

[j-bennet]

You also want to remove Python 2 and 2.7 from setup.py.

[laixintao]

Updated, sorry I forgot

[codecov-io]

Codecov Report

Merging #1153 into master will decrease coverage by 0.2%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1153      +/-   ##
==========================================
- Coverage   84.27%   84.07%   -0.21%     
==========================================
  Files          22       22              
  Lines        2531     2530       -1     
==========================================
- Hits         2133     2127       -6     
- Misses        398      403       +5

Impacted Files	Coverage Δ
pgcli/main.py	76.72% <ø> (-0.18%)	⬇️
pgcli/encodingutils.py	83.33% <0%> (-16.67%)	⬇️
pgcli/packages/sqlcompletion.py	97.35% <0%> (-0.34%)	⬇️
pgcli/pgexecute.py	82.28% <0%> (-0.31%)	⬇️

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[j-bennet]

👍

[j-bennet]

Merged! Thanks a lot! 🍫

[laixintao]

@j-bennet I am going to open a PR to drop 3.5 as well, and working on #1149