Drop support for Python 3.6

[edgarrmondragon]

Description of change

• Drop support for Python 3.6 using python_requires.
• Replace nose with pytest.
• Set up black and isort configurations (not yet applied to any project files).
• Create .pre-commit-config.yaml.

Manual QA steps

• Use pytest:

  python -m virtualenv venv
  source venv/bin/activate
  pytest
  

• I've used the GitHub CLI successfully with the command gh workflow run -r drop-python-3.6. Example output: https://github.com/edgarrmondragon/pipelinewise-singer-python/actions/runs/2302679477

Risks

• Packages using this library and relying on Python 3.6 might face problems if they try to upgrade without similarly dropping support for 3.6.

Rollback steps

• Revert this branch

Closes #63

[edgarrmondragon]

@Samira-El this is ready for review, lmk what you think!

[Samira-El]

Thanks Edgar for the PR!

In general it lgtm, I left few comments.

[edgarrmondragon]

@Samira-El thanks for the feedback. This is ready for another review :)

[Samira-El]

LGTM, thank you :)