Use .NET Core 3.1 LTS for tests (instead of 2.0)

[lostmsu]

What does this implement/fix? Explain your changes.

.NET Core 2.0 is no longer supported. This moves tests to .NET Core 3.1 LTS, which should be supported until Dec 2022.

[codecov-io]

Codecov Report

Merging #1279 (13694d6) into master (d0c588b) will increase coverage by 1.37%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1279      +/-   ##
==========================================
+ Coverage   86.25%   87.62%   +1.37%     
==========================================
  Files           1        1              
  Lines         291      291              
==========================================
+ Hits          251      255       +4     
+ Misses         40       36       -4     

Flag	Coverage Δ
setup_linux	64.94% <ø> (ø)
setup_windows	73.88% <ø> (+1.37%)	⬆️

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Impacted Files	Coverage Δ
setup.py	87.62% <ø> (+1.37%)	⬆️

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[lostmsu]

Travis actually passed, but for some reason was not recorded correctly: https://travis-ci.org/github/pythonnet/pythonnet/builds/743309873

Since there are no functional changes, and tests still pass, I am going to merge this.