Fix pyflakes tests for recent pyflakes versions

Summary:
Since 2.1.0 (commit 75bc0c03c145), pyflakes has included the Python
version and platform in its version output, so ignore it if present.

Since 2.2.0 (commit 6ba3f8e0b59b), pyflakes has included the column
number in its messages, so update the parser to include it and drop the
column number from the (only) test in order to work with both old and
new versions. Whilst here, assign names to the capture groups to make
the code clearer.

Test Plan: Ran arc unit

Reviewers: epriestley, joshuaspence, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D21504