coverage.py is broken on python3.8, upgrade to the latest version.

[laixintao]

Description

see dbcli/pgcli#1158

nedbat/coveragepy#714

Checklist

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

[laixintao]

https://travis-ci.org/github/dbcli/mycli/jobs/664080541#L736

[laixintao]

Can we get this merged :)

[codecov-io]

Codecov Report

Merging #838 into master will increase coverage by 26.18%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master     #838       +/-   ##
===========================================
+ Coverage   53.64%   79.82%   +26.18%     
===========================================
  Files          25       23        -2     
  Lines        3626     1750     -1876     
===========================================
- Hits         1945     1397      -548     
+ Misses       1681      353     -1328     

Impacted Files	Coverage Δ
mycli/main.py
mycli/__init__.py
mycli/packages/tabular_output/sql_format.py	100.00% <0.00%> (+2.32%)	⬆️
mycli/packages/special/delimitercommand.py	90.90% <0.00%> (+3.95%)	⬆️
mycli/packages/special/dbcommands.py	13.39% <0.00%> (+6.44%)	⬆️
mycli/config.py	88.46% <0.00%> (+10.75%)	⬆️
mycli/clitoolbar.py	33.33% <0.00%> (+16.31%)	⬆️
mycli/completion_refresher.py	95.45% <0.00%> (+18.62%)	⬆️
mycli/clistyle.py	64.00% <0.00%> (+22.44%)	⬆️
mycli/packages/special/iocommands.py	90.75% <0.00%> (+23.25%)	⬆️
... and 7 more

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