Annotation complexity should not fail on expressions

Bug report

This code:

def some() -> 'test expression':
    ...

Makes flake8-annotation-complexity to fail. We need to ignore this case silently.

Related: #1170

Demo: https://asciinema.org/a/IIjIfkVKytmZ1F5c2YufMORdi

[leksuss]

@sobolevn thank you for so useful answer!

[sobolevn]

@leksuss we cannot control why people would do that. But, we know that they will. And our goal is to help them not to make this mistake.

I appreciate your help! Thanks a lot! Since this one is the first PR for you, here are the steps:

1. You fork this repository: https://help.github.com/en/github/getting-started-with-github/fork-a-repo
2. Then (inside your fork) you add new failing tests here: https://github.com/wemake-services/wemake-python-styleguide/blob/master/tests/test_visitors/test_ast/test_complexity/test_annotation_complexity/test_annotation_complexity_nesting.py You goal at this point is to make sure that you can reproduce the bug. It is called ATDD
3. Then (inside your fork) you edit https://github.com/wemake-services/wemake-python-styleguide/blob/master/wemake_python_styleguide/logic/complexity/annotations.py and fix the bug.
4. Then you run your tests again and they should pass this time!
5. You submit a PR: https://opensource.com/article/19/7/create-pull-request-github
6. Done!

You can find the information about running tests here: https://github.com/wemake-services/wemake-python-styleguide/blob/master/CONTRIBUTING.md