Fix compatibility with Flynt 0.78 and 1.0.0

[akaihola]

Fixes #472.

We temporarily limited Flynt to version 0.77 and earlier in version 1.7.1 (see #474). This patch fixes compatibility with Flynt 0.78 and 1.0.x.

• [ ] merge #501 (to minimize changes on parts which will be changed in that PR)
• [ ] rebase on master
• [ ] drop support for Python 3.7 (strange flynt related test failure only on 3.7)
• [ ] address __main__() complexity as reported by ruff

[baodrate]

note that since this PR was started, flynt v1.0.0 has been released: https://github.com/ikamensh/flynt/releases/tag/1.0.0