Update CI to ubuntu 20.04

[iMichka]

No description provided.

[codecov-commenter]

Codecov Report

Merging #156 (547afd2) into develop (27aa1a8) will decrease coverage by 0.04%.
The diff coverage is 0.00%.

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@@             Coverage Diff             @@
##           develop     #156      +/-   ##
===========================================
- Coverage    91.74%   91.70%   -0.04%     
===========================================
  Files          138      138              
  Lines        10723    10723              
===========================================
- Hits          9838     9834       -4     
- Misses         885      889       +4     

Flag	Coverage Δ
unittests	91.70% <0.00%> (-0.04%)	⬇️

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Files Changed	Coverage Δ
pygccxml/parser/source_reader.py	85.71% <0.00%> (ø)
unittests/project_reader_correctness_tester.py	82.22% <ø> (ø)

... and 2 files with indirect coverage changes