I'm not sure where this code went or when, but it disappeared. Let's
add it back.

Related #180

Handle multiline strings with '# noqa'

*Description of changes*

I had overlooked a usecase of Flake8 where people use `# noqa` at the end of a multi-line string. This addresses that oversight

*Related to:*  #177

See merge request !85

In Flake8 2.x we allowed people to use # noqa at the end of a multiline
string to ignore errors inside the string (e.g., E501). Being blissfully
ignorant of this, I never accounted for it in Flake8 3. This fixes the
oversight and allows multiline statements to have the # noqa at the end.

Closes #177

Force flake8 test to below 3.x

The current version (0.8) of `flake8-import-order` still uses `parser.config_options` and so all builds fail because it installs Flake8 3.x.

Now the repository of that plugin already accounted for that but as long as it isn't released it'll cause all new builds to fail. Alternatively I can repurpose this merge request to actually enforce a newer version of `flake8-import-order` which supports Flake8 3.x.

See merge request !84

Because flake8-import-order does not support Flake8 3.x yet, we need to force
a Flake8 version that is supported.

It is possible to write plugins which are only a function. At the moment they
are called on each line manually. This allows the function also to be called
on each file once. It works similar to creating the class and calling `run` on
it immediately. The plugin function needs to return a generator.

This is based on the original comment in the `FileChecker.run_ast_checks`
method, but slightly modified as the original comment would've called the
return of the function. But the function could return the reports directly.

Check for both os.path.sep and os.path.altsep

*Description of changes*

When normalizing paths, we want to handle the following cases:

- Someone is using a Windows-style path on Windows
- Someone is using a Unix style path on Unix
- Someone is using a Unix style path on Windows

os.path.sep will handle the native directory separator character while os.path.altsep (when set) will handle alternate separators. Further,  os.path.abspath does the right thing on Windows when handed a Unix-style path.

*Related to:*  #175

See merge request !81