Maintainership converter command line tool

[ncounter]

Fixes openSUSE/openSUSE-git#311

• convert from legacy to latest format
• print the produced content to the CLI output

How to use it

Run the git-obs maintainership-converter command, it reads the _maintainership.json file from the cwd and print to CLI output the file content converted to the new format.
The user can optionally specify the path/to/file to convert as a parameter: git-obs maintainership-converter _maintainership.json

[codecov]

Codecov Report

❌ Patch coverage is 90.90909% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 33.96%. Comparing base (ece94c9) to head (b22d9f1).
⚠️ Report is 122 commits behind head on master.

Files with missing lines	Patch %	Lines
osc/commands_git/maintainership_converter.py	90.90%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2107      +/-   ##
==========================================
- Coverage   34.11%   33.96%   -0.16%     
==========================================
  Files         189      197       +8     
  Lines       25889    26974    +1085     
==========================================
+ Hits         8833     9162     +329     
- Misses      17056    17812     +756     

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