This adds support for rules that ignore violation codes on a per-file
basis. This takes a similar functional approach to
https://github.com/snoack/flake8-per-file-ignores which allows for glob
patterns like the `--exclude` option.

Closes #156

Instead of just using Flake8 and pylint to keep Flake8 clean, let's also
use black to make it less manual for clean-up.

Fix docs for --isolated flag

See merge request pycqa/flake8!247

(I ran this 5 times and picked a median result)

### before

```console
$ time flake8 --help > /dev/null

real    0m0.221s
user    0m0.199s
sys     0m0.020s
```

### after

```console
$ time flake8 --help > /dev/null

real    0m0.169s
user    0m0.144s
sys     0m0.020s
```

The URL https://docs.python.org/3/ is actively maintained and updated.

Refs #365

This includes creating the pool, tearing it down under normal use, and
tearing it down in case of exception.

Doing this makes it harder to leak processes, as for instance was
happening in #410.

Fixes #410

fix for flake8-isort

See merge request pycqa/flake8!215