Add a template for GHSAs (#158612)

Co-authored-by: Ezio Melotti <ezio.melotti@gmail.com>

Author: StanFromIreland

.github/VULNERABILITY_REPORT.yml

@@ -0,0 +1,73 @@
+name: Vulnerability report
+description: Privately report a potential security vulnerability in CPython
+body:
+  - type: markdown
+    attributes:
+      value: |
+        **Not all bugs are vulnerabilities.** Before submitting, read the [Python security policy](https://devguide.python.org/security/policy/) to understand which issues are vulnerabilities and what versions of Python accept reports.
+
+        Keep the report short and in plain text: no headers, tables, PDFs, binaries, or severity and CVSS information.
+
+        Reports that do not contain a potential security vulnerability will be discarded without a reply.
+
+        To report vulnerabilities that affect other projects (such as pip or python.org), or if you are not sure where to send your report, email [security@python.org](mailto:security@python.org).
+  - type: textarea
+    id: summary
+    attributes:
+      label: Summary
+      description: A few sentences describing the vulnerability.
+    validations:
+      required: true
+  - type: textarea
+    id: threat_model
+    attributes:
+      label: Threat model
+      description: >
+        What does the attacker control, and what do they gain? Describe the code, configuration, or deployment that may exist in the real world and is exploitable. Where possible, cite the relevant part of the [security policy](https://devguide.python.org/security/policy/#what-types-of-bugs-are-vulnerabilities).
+    validations:
+      required: true
+  - type: textarea
+    id: proof_of_concept
+    attributes:
+      label: Proof of concept
+      description: >
+        A script that reproduces the issue and clearly indicates whether the vulnerability is present, such as exiting with `1` if vulnerable and `0` if not. If it depends on a specially constructed binary file, include a script to construct the file rather than the file itself.
+
+        Wrap scripts longer than a few lines in a [collapsed section](https://docs.github.com/en/get-started/writing-on-github/working-with-advanced-formatting/organizing-information-with-collapsed-sections) using `<details> ... </details>`.
+    validations:
+      required: true
+  - type: dropdown
+    id: versions
+    attributes:
+      label: "CPython versions tested on:"
+      description: >
+        If any tested version was not vulnerable, say which in the summary. Only [supported versions](https://devguide.python.org/versions/) accept reports.
+      multiple: true
+      options:
+        - "3.11"
+        - "3.12"
+        - "3.13"
+        - "3.14"
+        - "3.15"
+        - "3.16"
+        - "CPython main branch"
+    validations:
+      required: true
+  - type: textarea
+    id: patch
+    attributes:
+      label: Suggested fix
+      description: Ideally, a minimal patch with the mitigation.
+    validations:
+      required: false
+  - type: checkboxes
+    id: checklist
+    attributes:
+      label: Before submitting
+      options:
+        - label: I have read the security policy and evaluated this report against it.
+          required: true
+        - label: I have checked that this issue is not already resolved on the `main` branch.
+          required: true
+        - label: I have verified the factual validity of everything in this report, including any content produced by an LLM.
+          required: true