🔖 Prepare release of v2.1.0

[denialhaag]

Description

This PR prepares the release of v2.1.0.

Checklist:

• The pull request only contains commits that are focused and relevant to this change.
• I have added appropriate tests that cover the new/changed functionality.
• I have updated the documentation to reflect these changes.
• I have added entries to the changelog for any noteworthy additions, changes, fixes, or removals.
• I have added migration instructions to the upgrade guide (if needed).
• The changes follow the project's style guidelines and introduce no new warnings.
• The changes are fully tested and pass the CI checks.
• I have reviewed my own code changes.

Summary by CodeRabbit

• Documentation
  • Added 2.1.0 release notes with upgrade guidance and a dated entry (2025-10-15).
  • Announced support for Qiskit IfElse and Python 3.14 testing; noted end-of-support for Python 3.9.
  • Updated comparison and release links to reference the 2.1.0 tag and refreshed Unreleased section.
  • Harmonized formatting and clarified the supported Python versions.

[coderabbitai]

📝 Walkthrough

Walkthrough

Adds 2.1.0 release notes and upgrade guidance: CHANGELOG.md gains a 2.1.0 section noting IfElse support and Python 3.14 testing, updates minimum mqt-core and removes Python 3.9 support; UPGRADING.md adds a 2.1.0 note about Python 3.9 end-of-support and updates compare URLs to the ddsim repo.

Changes

Cohort / File(s)

Summary of Changes

Release notes & upgrade guidance CHANGELOG.md, UPGRADING.md

Added 2.1.0 sections. CHANGELOG.md: new 2.1.0 header dated 2025-10-15, upgrade note linking to UPGRADING.md#210, Added entries for Qiskit IfElse support and Python 3.14 testing, Changed minimum mqt-core to 3.3.1, Removed Python 3.9 support, updated comparison links (v2.1.0...HEAD) and added [2.1.0] tag link. UPGRADING.md: added 2.1.0 heading noting Python 3.9 end-of-support and updated versioned compare URLs to point to the ddsim repository.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Poem

I’m a rabbit in the changelog burrow, bright—🐇
2.1.0 hops in, links set right.
Python 3.14 gives a cheerful wink,
3.9 nods off softly, taking a blink.
Docs aligned, we leap tonight. ✨

Pre-merge checks and finishing touches

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Description Check	⚠️ Warning	The PR description provides a brief summary and a checklist but omits required context and dependencies, and the checklist does not accurately reflect the documentation and changelog updates that were performed.	Update the description to include relevant motivation and context for the release, list any dependencies if applicable, ensure the “Fixes” line is removed or added correctly, and mark the documentation and changelog update checklist items to reflect the actual changes made.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Title Check	✅ Passed	The title succinctly captures the primary change of preparing the v2.1.0 release and is fully related to the changeset, but it includes an emoji which can be considered noise according to conventional title guidelines.
Docstring Coverage	✅ Passed	No functions found in the changes. Docstring coverage check skipped.

✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch prepare-release-2-1-0

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📜 Recent review details

Configuration used: CodeRabbit UI

Review profile: ASSERTIVE

Plan: Pro

📥 Commits

Reviewing files that changed from the base of the PR and between 2767925 and e14d3d2.

📒 Files selected for processing (1)

• CHANGELOG.md (2 hunks)

🧰 Additional context used

🪛 LanguageTool

CHANGELOG.md

[grammar] ~12-~12: Use correct spacing
Context: ...## [Unreleased] ## [2.1.0] - 2025-10-15 _If you are upgrading: please see [`UPGRA...

(QB_NEW_EN_OTHER_ERROR_IDS_5)

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[grammar] ~14-~14: Use colons correctly
Context: ...] ## [2.1.0] - 2025-10-15 _If you are upgrading: please see [UPGRADING.md](UPGRADING.m...

(QB_NEW_EN_OTHER_ERROR_IDS_30)

---

[grammar] ~14-~14: Use correct spacing
Context: ... see UPGRADING.md._ ### Added - ✨ Add support for Qiskit's `IfE...

(QB_NEW_EN_OTHER_ERROR_IDS_5)

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_{Comment @coderabbitai help to get the list of available commands and usage tips.}

[coderabbitai]

Actionable comments posted: 1

Caution

Some comments are outside the diff and can’t be posted inline due to platform limitations.

⚠️ Outside diff range comments (1)

CHANGELOG.md (1)

12-21: Add upgrade pointer for 2.1.0

2.0.0 already nudges readers to the upgrade guide; doing the same here helps highlight the Python 3.9 removal for anyone scanning release notes.

Apply this diff to add the pointer:

 ## [2.1.0] - 2025-10-15
+
+_If you are upgrading: please see [`UPGRADING.md`](UPGRADING.md#210)._

📜 Review details

Configuration used: CodeRabbit UI

Review profile: ASSERTIVE

Plan: Pro

📥 Commits

Reviewing files that changed from the base of the PR and between db93014 and 69c77a0.

📒 Files selected for processing (2)

• CHANGELOG.md (2 hunks)
• UPGRADING.md (2 hunks)

🧰 Additional context used

🪛 LanguageTool

CHANGELOG.md

[grammar] ~12-~12: Use correct spacing
Context: ...## [Unreleased] ## [2.1.0] - 2025-10-15 ### Added - 👷 Enable testing on Python 3.1...

(QB_NEW_EN_OTHER_ERROR_IDS_5)

UPGRADING.md

[grammar] ~7-~7: Use correct spacing
Context: ...ANGELOG.md). ## [Unreleased] ## [2.1.0] ### End of support for Python 3.9 Starting ...

(QB_NEW_EN_OTHER_ERROR_IDS_5)