Guard against breaking changes in isort'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>=21.5b1
    isort>=5.0.1

We use internal Python APIs from both 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 both Black and isort, 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

Since Darker 1.4.1, we now do guard against backwards incompatible internal changes in Black 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 release. See #186 and #284 for details.

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

We should thus either

• pin isort to a fixed version in action/main.py, or
• install isort from its main branch as well in action/main.py.