deprecations setuptools

[mdonadoni]

• ci(pytest): invoke pytest directly instead of setup.py test (deprecations setuptools #466)
• build(python): remove deprecated pytest-runner (deprecations setuptools #466)
• build(python): use optional deps instead of tests_require (deprecations setuptools #466)
• build(python): add minimal pyproject.toml (deprecations setuptools #466)

Closes reanahub/reana#814

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 36.31%. Comparing base (e37f272) to head (9468850).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #466   +/-   ##
=======================================
  Coverage   36.31%   36.31%           
=======================================
  Files          26       26           
  Lines        1578     1578           
=======================================
  Hits          573      573           
  Misses       1005     1005