Guard against breaking changes in Flynt's Python API

[akaihola]

The GitHub Action script currently allows the user to pin Darker itself to a given version using the version: option (see action/main.py).

Darker's dependencies, however, only specify minimum versions in install_requires and extras_require in setup.cfg, such as:

    black>=21.7b1
    flynt>=0.76
    isort>=5.0.1

We use internal Python APIs from each of those packages. Black doesn't even define a public Python API (yet; see psf/black#779). We use Python APIs instead of calling these tools as subprocesses for performance and flexibility reasons.

Breaking changes have occurred in the past for Black, isort and Flynt, see:

• Add support of isort>5 #16
• Read configuration from pyproject.toml #58 and Move to black 20.8b1 #62
• Mypy & Black 21.x compatibility #135
• Fix gen_python_files usage for black>=21.7b1.dev9 #189
• incompatible with black v22.1.0 ? #264 and Add compatibility with Black >= 22.1 #270
• flynt checks breaking with package update to flynt-0.78 #472

Since Darker 1.4.1, we do guard against backwards incompatible internal changes in Black and isort by running a test-future workflow weekly and also as part of push triggered builds. So we will get an early warning if it seems an incompatibility is coming up in the next Black or Flynt release. See #186 and #284 for details.

For flynt we don't currently have the same mechanism in place.

We should thus add flynt@main to be used in the test-future workflow, similar to what was done for isort in #296.