Pick lint from downstream_issue.py

[webbnh]

This PR is the last in the series of revisions originally set out in #207 which endeavors to improve code quality in Sync2Jira; this is the follow-on to #261.

This change picks lint from the downstream-issues module:

• adds some type hints to resolve incorrect type inference in the IDE
• makes editorial corrections in code comments
• removes unneeded escape characters from a character-set in a regular expression
• replaces camelCase with snake_case
• renames identifiers whose declarations shadow identifiers from outer scopes
• adds a missing function parameter description
• removes an unused function parameter

Also this PR contains two changes for the _close_as_duplicate() function:

• adds text to duplicated log messages to disambiguate them
• adds a guard to a condition exception handler to avoid accesses to nonexistent attributes