Fix local dependencies and update documentation

[FedeG]

Changes

• Fix django-silk version: use PyPi package with specific version
• Update readme
  • Add syntax language to code blocks
  • Add create superuser documentation
  • Use venv python module instead of virtualenv
• Add python 3.9 to black target version
• Run black over manage.py

Issues

• Fix Django-silk version on local.txt failed #135

[FedeG]

@devrubens could you merge this pull request?
If you try to install this module (dev branch) in an empty environment, the dependencies are broken and can't run it.
In this pull request, I fixed it.