[AIRFLOW-5205] Check xml files with xmllint + Licenses

[potiuk]

Make sure you have checked all steps below.

Jira

• My PR addresses the following Airflow Jira issues and references them in the PR title. For example, "[AIRFLOW-XXX] My Airflow PR"
  • https://issues.apache.org/jira/browse/AIRFLOW-5205

Description

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Tests

• My PR adds the following unit tests OR does not need testing for this extremely good reason:

Commits

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  1. Subject is separated from body by a blank line
  2. Subject is limited to 50 characters (not including Jira issue reference)
  3. Subject does not end with a period
  4. Subject uses the imperative mood ("add", not "adding")
  5. Body wraps at 72 characters
  6. Body explains "what" and "why", not "how"

Documentation

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Code Quality

• Passes flake8

[Fokko]

What's the point of opening this PR? It is impossible to review with some many dependencies on other PRs.

[feluelle]

Agree with Fokko. I personally don't want to be responsible for reviewing such a huge PR and maybe miss something - but I really like reviewing PR's in general :(

@potiuk do you think it is possible to split it into smaller ones?

[Fokko]

I've noticed that the PR's in the list are sequential, it starts with this one: #5777

[potiuk]

@Fokko @feluelle : apologies for that. i split those changes into smaller, fairly independent commits (hence the "depends on" in the PR title). And it was much easier to do the "series" of commits. They all depend on the #5777 and once I merge that one (Thanks @Fokko for the approaval), I will actually change those commits/PR and make them totally separate (and not depending on each other).

[potiuk]

Made the PR standalone (not depending on series of PRs

[potiuk]

This one is really small and mostly about consistency in licence. Rather straightforward (the only potential problem is necessity of xmllint - but this is really not a problem because pre-commit will skip that check if you don't modify any of the xml files - so you won't need xmllint in most of the cases.