No more scripts!

[aleksul]

Resolves #58.

Main changes:

• DELETE scripts
• DELETE frozen requirements, instead ADD dev and test requirements in setup.py
• Instead of CI linting and formatting checks, ADD pre-commit hooks
• ADD tox for better testing
• UPDATE CI to test against multiple python versions and different OS
• UPDATE contributing guide

[aleksul]

It should also be noted that this solution is inspired by other repositories from pytest-dev.