Add Ubuntu 20.04

[exarkun]

https://tahoe-lafs.org/trac/tahoe-lafs/ticket/3328

I started off dropping 16.04 as well but then I realize it's LTS for a while longer so we shouldn't be too casual about dropping it.

[codecov-commenter]

Codecov Report

Merging #768 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #768      +/-   ##
==========================================
+ Coverage   91.24%   91.26%   +0.01%     
==========================================
  Files         155      155              
  Lines       27223    27223              
  Branches     3871     3871              
==========================================
+ Hits        24839    24844       +5     
+ Misses       1662     1659       -3     
+ Partials      722      720       -2     

Impacted Files	Coverage Δ
src/allmydata/web/status.py	94.92% <0.00%> (+0.22%)	⬆️
src/allmydata/mutable/servermap.py	94.22% <0.00%> (+0.48%)	⬆️

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[sajith]

Yay, tests pass!

[sajith]

Curious, why is this requires used here? Does this mean that we don't want to test on Ubuntu 18.04 if testing on Ubuntu 20.04 fails?

[exarkun]

Yea, essentially. In a perfect world I don't think we'd do this but in a highly CI-resource-constrained world I think it makes sense. I'm sure Ubuntu 20.04 working is not perfectly correlated with Ubuntu 18.04 working but my guess is it's pretty highly correlated and that often the failures are the same. So don't bother spending CI resources on 18.04 if 20.04 is broken. It would be pretty interesting to collect some data on this but I haven't.