Travis CI: 'sudo' tag is now deprecated in Travis

[cclauss]

Travis are now recommending removing the sudo tag.

[jeking3]

You might be able to move dist: xenial out one level and define it globally. If so, you can move the python declarations out to the global list and eliminate the matrix include section all together.

[cclauss]

I can confirm that both solutions work. While xenial is generally available, it is not the Travis default distro so I usually take the more conservative route of only using it where it is needed.

[cclauss]

#831 eliminates one test when run on xenial so I think it is best to run more tests on Pythons that are compatible with the Travis CI default distro and fewer tests only on Python 3.7 and nightly.

[codecov-io]

Codecov Report

Merging #825 into master will decrease coverage by 0.11%.
The diff coverage is n/a.

@@ Coverage Diff @@## master #825 +/- ##
==========================================
- Coverage 94.79% 94.67% -0.12% 
==========================================
Files 59 59 Lines 9600 9386 -214 ==========================================
- Hits 9100 8886 -214 
Misses 500 500

Impacted Files	Coverage Δ
git/objects/tree.py	59.64% <0%> (-0.93%)	⬇️
git/index/util.py	90.69% <0%> (-0.61%)	⬇️
git/index/typ.py	96.82% <0%> (-0.58%)	⬇️
git/refs/log.py	93.84% <0%> (-0.53%)	⬇️
git/objects/base.py	95.83% <0%> (-0.33%)	⬇️
git/refs/tag.py	96.66% <0%> (-0.31%)	⬇️
git/test/lib/helper.py	91.86% <0%> (-0.28%)	⬇️
git/refs/reference.py	95.74% <0%> (-0.26%)	⬇️
git/objects/commit.py	91.59% <0%> (-0.25%)	⬇️
git/cmd.py	83.83% <0%> (-0.18%)	⬇️
... and 26 more

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