This updates Pydocstyle8Tool to subclass the newer BaseTool, and to

use the newer features it offers (dependencies, improved configurable

file extension handling, centralized base command building, and

standardized error code output).
The new version of the tool has corrected report parsing. The old

version reported the context of a failure ("in public module", etc.),

rather than the actual error. This was partly because pydocstyle error

reporting spans multiple lines for a single match.
Now, we use a multi-line regex to match this, and extract the correct

information.
Unit tests were added to cover the new pydocstyle parsing, and to

provide integration tests.

Unit tests pass on Python 2.7 and 3.x.