1047 modernize ci

[MartinBubel]

Move from travis et al to github actions for testing and deploy.

There still are some leftovers that can be adressed in an upcoming polishing phase.

Closes #1047

[MartinBubel]

@lawrennd I added a github actions file to replace the old-styled travis ci.
This allows us for building and testing GPy on windows, linux, and macos CI runners, within the policies of an open source project (so no additional cost for CI jobs).

Deploying linux and macos wheels should address some of the compatibility issues.

There might be some leftovers for polishing but I suggest we get this done as quickly as possible.
Please approve the PR if this is fine for you.

[codecov]

Codecov Report

Merging #1053 (eb610cc) into devel (328312f) will decrease coverage by 1.63%.
Report is 2 commits behind head on devel.
The diff coverage is 77.77%.

❗ Current head eb610cc differs from pull request most recent head ac15eac. Consider uploading reports for the commit ac15eac to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##            devel    #1053      +/-   ##
==========================================
- Coverage   51.68%   50.05%   -1.63%     
==========================================
  Files         210      210              
  Lines       22198    22197       -1     
  Branches     3314     3314              
==========================================
- Hits        11473    11111     -362     
- Misses      10187    10563     +376     
+ Partials      538      523      -15     

[lawrennd]

Thanks Martin!

[lawrennd]

Thanks Martin!