Add support to testing and debugging with pytest on VSCode

[juarezr]

This PR has the objective of integrating recently migrated pytest into VSCode

Changes

1. Removed nosetests hacks
2. Enabled testing and debugging with pytest directly on VSCode
3. Added VSCode tasks for package build and install

Also:

• These changes only affect developers using VSCode.
• Detection is automatic when opening the local folder from the git clone

Checklist

Checklist for for pull requests including new code and/or changes to existing code...

• [ ] Code
  • [ ] Includes unit tests
  • [ ] New functions have docstrings with examples that can be run with doctest
  • [ ] New functions are included in API docs
  • [ ] Docstrings include notes for any changes to API or behaviour
  • [ ] All changes documented in docs/changes.rst
• [ ] Testing
  • [ ] (Optional) Tested local against remote servers
  • [ ] Github CI passes (unit tests run under Linux/Windows/MacOS)
  • [ ] AppVeyor CI passes (unit tests run under Windows)
  • [ ] Unit test coverage has not decreased (see Coveralls)
• [ ] Changes
  • [ ] (Optional) Work in progress
  • [X] Ready to review
  • [X] Ready to merge

[coveralls]

Coverage remained the same at 90.871% when pulling ea65743fdfa94c968e0cb56b3fbd12581158167a on juarezr:vscode_pytest into 860a333d8b3290a421c5207c747537b13129017e on petl-developers:master.