Add License to headers

[juanitorduz]

We need to add a license to all our Python files. We can use https://github.com/lucianopaz/head_of_apache as in

https://github.com/pymc-devs/pymc/blob/881030ab330e3aa21df67ae3f243405eb3747c07/.pre-commit-config.yaml#L19-L28

- repo: https://github.com/lucianopaz/head_of_apache
  rev: "0.0.3"
  hooks:
    - id: head_of_apache
      args:
        - --author=The PyMC Developers
        - --exclude=docs/
        - --exclude=scripts/
        - --exclude=binder/
        - --exclude=versioneer.py