Use Darker as code formatter. Darker is:
"""
Apply black reformatting to Python files only in regions changed since a given commit.
"""

Also use `pytest-darker`, `pytest-flake8` and `pytest-isort` ... So code formatting "errors" will be
check in pytest run. (Note: Only the first test is slower, afer one test runs only changed files
will be checked.)

Update requirements

Use darker as code formatter tool base...

#70 showing task description when available on TaskAdmin

In practice, the hostname can change constantly. In that case, the list is very long. Move it to the
end. Then it does not bother so much.

Maybe we should remove/add this field, by settings?

Lower max line length to 100.
Exclude migration files.

Enhancements

Update requirements and README and prepare v0.4.6 release

Code check is done in pytest run, since: #74

And there is a problem with 'fix-code-style', because of:
akaihola/darker#273 (black would like to access docker volumes and raise a
permission error)