deprecations setuptools

[mdonadoni]

• ci(pytest): invoke pytest directly instead of setup.py test (deprecations setuptools #725)
• build(python): remove deprecated pytest-runner (deprecations setuptools #725)
• build(python): use optional deps instead of tests_require (deprecations setuptools #725)
• build(python): add minimal pyproject.toml (deprecations setuptools #725)

Closes reanahub/reana#814

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 58.59%. Comparing base (822a538) to head (166ced7).
Report is 15 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #725   +/-   ##
=======================================
  Coverage   58.59%   58.59%           
=======================================
  Files          24       24           
  Lines        2391     2391           
=======================================
  Hits         1401     1401           
  Misses        990      990