ArcanistPyLintLinter fix to getMandatoryFlags msg-template

Summary: Removed excess quotations on the --msg-template option as it was interfering with the string-int coercion

Test Plan: Unsure

Reviewers: joshuaspence, epriestley, Blessed Reviewers

Reviewed By: joshuaspence, epriestley, Blessed Reviewers

Subscribers: joshuaspence, e-m-albright, Korvin

Maniphest Tasks: T9214

Differential Revision: https://secure.phabricator.com/D13931