docs: Update changelog

[vdusek]

Updates CHANGELOG.md: bumps the unreleased section to 4.0.0 to reflect the breaking change (dropping Python 3.10 support, #908) and adds the 3.4.1 release section.

The version bump is needed because the unreleased changes include a breaking change, so the next release must be a major version.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.04%. Comparing base (eb0323d) to head (a7c24a9).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #916      +/-   ##
==========================================
+ Coverage   87.00%   87.04%   +0.03%     
==========================================
  Files          48       48              
  Lines        2956     2956              
==========================================
+ Hits         2572     2573       +1     
+ Misses        384      383       -1     

Flag	Coverage Δ
e2e	37.58% <ø> (ø)
integration	58.93% <ø> (+0.03%)	⬆️
unit	75.81% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.