upgrade coverage to 5.x

[laixintao]

Description

This fixes the coverage issue from #1157 , it tunes out that the version of coverage.py we are using is broken on Python3.8, which produces some phantom lines. see nedbat/coveragepy#714 (which is actually interesting).

So simply upgrade coverage.py solves the problen.

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)

[laixintao]

The line data from @pasenor 's script draws my attention( #1157 (comment) ), I don't the difference is normal, but the script is correct and I even checked the lines manually.

I figured it out by running coverage locally, this is html report from Python3.6 pgcli/main.py:

And this from Python3.8:

Then google lead me to nedbat/coveragepy#714

[j-bennet]

@laixintao Good catch!