ci: upgrade to flit 4.0.2

[ricellis]

PR summary

• Update pyproject.toml [build-system] from ["flit_core >=3.9,<4"] to ["flit_core >=4.0.2,<5"]
• Upgrade flit dependency in requirements-dev.txt to 4.0.2

Fixes: s1360

PR Checklist

Please make sure that your PR fulfills the following requirements:

• The commit message follows the
  Angular Commit Message Guidelines.
• Tests for the changes have been added (for bug fixes / features) - N/A build
• Docs have been added / updated (for bug fixes / features) - N/A build

PR Type

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• New tests
• Build/CI related changes
• Documentation content changes
• Other (please describe)

What is the current behavior?

Uses flit 3.12.0

What is the new behavior?

Uses flit 4.0.2

Does this PR introduce a breaking change?

• Yes
• No

Other information

Requires CI to run in an env with flit 4.0.2 available.