This attribute is introduced in pycodestyle 2.1.0

Closes #246

See: https://github.com/PyCQA/pycodestyle/issues/400

Apparently, some folks pass an empty string to --stdin-display-name. To
avoid the errors this causes, we need to handle it appropriately.

Closes #235

We opted to not copy the file_tokens attribute each time it's accessed
in the merge request discussion but it was never reflected in the code.
Further, the attribute had no documentation or docstring, so we've added
that. Finally, we address a personal style nit that I otherwise wouldn't
have picked at.

Closes #205

Serialize Checkers PluginTypeManager to a dict

*Description of changes*

Try to side-step issues with attributes not being set/updated on plugins when used with multiprocessing and Queues.

*Related to:*  #179, #164 

See merge request !98

...since on py3 tokenize.TokenError does not have a 'message' attribute.
See #203.

It seems likely that the multiprocessing module on Windows is not
capable of serializing an object with the structure that we have and
preserving the attributes we dynamically set on plugins (like the
FlakesChecker). To avoid issues like this with all plugins (although
we have only found this on Windows with the FlakesChecker), let's try
serializing the Checkers PluginTypeManager to a dictionary so that the
only object that a Queue is really trying to serialize/deserialize is
the FlakesChecker itself.

Related to #179

When attempting to centralize all inline NoQA handling in the StyleGuide
we inadvertently broke plugins relying on it in combination with checker
state. For example, the check for E402 relies both on NoQA and the state
to determine if it has seen a non-import line. Placing NoQA on the sole
line that is not an import is more elegant than placing it on each of
the following import lines.

Closes #186