Support flake8 4.0.0

[dycw]

Describe the bug
flake8 4.0.0 was released earlier today (2021-10-11), and I noticed this is one of several plugins not supporting 4.0.0. This should be updated.

Discovered whilst updating pre-commit hooks:

stderr:
      DEPRECATION: A future pip version will change local packages to be built in-place without first copyin
g to a temporary directory. We recommend you use --use-feature=in-tree-build to test your packages with this
 new behavior before it becomes the default.
       pip 21.3 will remove support for this functionality. You can find discussion regarding this at https:
//github.com/pypa/pip/issues/7555.
    ERROR: Cannot install flake8 4.0.0 (from /home/wande/.cache/pre-commit/repo1s37k6t_), flake8-absolute-im
port==1.0, flake8-annotations==1.0.0, flake8-annotations==1.1.0, flake8-annotations==1.1.1, flake8-annotatio
ns==1.1.2, flake8-annotations==1.1.3, flake8-annotations==1.2.0, flake8-annotations==2.0.0, flake8-annotatio
ns==2.0.1, flake8-annotations==2.1.0, flake8-annotations==2.2.1, flake8-annotations==2.3.0, flake8-annotatio
ns==2.4.0, flake8-annotations==2.4.1, flake8-annotations==2.5.0, flake8-annotations==2.6.0, flake8-annotatio
ns==2.6.1 and flake8-annotations==2.6.2 because these package versions have conflicting dependencies.
    ERROR: ResolutionImpossible: for help visit https://pip.pypa.io/en/latest/user_guide/#fixing-conflicting
-dependencies

To Reproduce
Minimal code example to reproduce the behavior:

python3 -m venv flake8-annotations
cd flake8-annotations/
source bin/activate                               
pip install flake8==4 flake8-annotations

[sco1]

Should just be a spec bump, will have a release soon.