feat: Backup of repository security advisories

[lukasbestle]

About the feature

This PR adds a new --security-advisories argument. This argument is included in --all.

When enabled, it grabs the repository security advisories and dumps them into a security-advisories directory.

This implements #243.

Implementation

I've copied and adapted the existing implementation of backup_milestones to backup_security_advisories as the underlying GitHub API structure is basically identical.

Advisories are indexed by their ghsa_id (GitHub Security Advisory ID), which is used as a primary key in the GitHub web URLs and also the way these advisories are commonly referred to.

Testing

I've tested the code on macOS with a public repo that contains both open and closed advisories.

[josegonzalez]

@Iamrodos mind helping @lukasbestle figure out how to fix this/maybe add a test case for this too?

[Iamrodos]

@Iamrodos mind helping @lukasbestle figure out how to fix this/maybe add a test case for this too?

Will do. Can get to it in a few hours later today. @lukasbestle you can give me access to the PR, otherwise I will just attach a patch file you can add which adjusts the tests. I can also add a new test for this.

[Iamrodos]

@lukasbestle here is a patch you can apply which will add the new argument to the tests.
fix-test-for-security-advisories.patch

Download the file and git apply fix-test-for-security-advisories.patch

OR

Edit tests/test_all_starred.py and add this line after line 39 (args.include_milestones = False):

    args.include_security_advisories = False

After this PR is merged, I will submit a new PR that makes the tests less fragile when a new parameter is added. I hit this myself on some of my other PRs and had it as a thing to fix.

I might also submit a PR template that includes

  ## Test plan
  - [ ] Tests pass locally (`pytest`)
  - [ ] Linting passes (`flake8 --ignore=E501`)

[lukasbestle]

Thanks for your super quick replies and the patch.