deprecations setuptools

[mdonadoni]

• ci(pytest): invoke pytest directly instead of setup.py test (deprecations setuptools #71)
• build(python): remove deprecated pytest-runner (deprecations setuptools #71)
• build(python): use optional deps instead of tests_require (deprecations setuptools #71)
• build(python): add minimal pyproject.toml (deprecations setuptools #71)

Closes reanahub/reana#814

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (a93ff15) to head (a93c1d6).

Additional details and impacted files

@@            Coverage Diff            @@
##            master       #71   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         2           
  Lines            7         5    -2     
=========================================
- Hits             7         5    -2     

see 2 files with indirect coverage changes