Fix failing pre-commit hook fails on directories

[letmerecall]

Purpose

Why this PR?
When the change is in directory, pre-commit hook fails with error.

Adding a symlink in text/symlink raise this error:

Check Yaml...........................................(no files to check)Skipped
Format code (black)......................................................Passed
Sort imports (isort).....................................................Passed
Check for PII............................................................Failed
- hook id: pii-check
- duration: 0.11s
- exit code: 1

Traceback (most recent call last):
  File "/home/guydumais/.cache/pre-commit/repomg4xos05/py_env-python3.8/bin/pii_check", line 8, in <module>
    sys.exit(main())
  File "/home/guydumais/.cache/pre-commit/repomg4xos05/py_env-python3.8/lib/python3.8/site-packages/pii_check/pii_check_hook.py", line 155, in main
    check_for_pii(args.url, API_KEY, enabled_entity_list)
  File "/home/guydumais/.cache/pre-commit/repomg4xos05/py_env-python3.8/lib/python3.8/site-packages/pii_check/pii_check_hook.py", line 104, in check_for_pii
    flagged = get_flagged_lines(files)
  File "/home/guydumais/.cache/pre-commit/repomg4xos05/py_env-python3.8/lib/python3.8/site-packages/pii_check/pii_check_hook.py", line 36, in get_flagged_lines
    with open(file, "r") as fp:
IsADirectoryError: [Errno 21] Is a directory: 'text/symlink'

The error is a result of checking the PII_CHECK flag on changed files and dirs.

Description

What is changed?

• Skip PII_CHECK flag check for directories.
• Add unittest for get_flagged_lines()

How has this been tested?

• Running unittest.
• Installing updated version of pre-commit hook and testing on the failing case.

Checklist

• No linting error?
• No dead code?

[guyd]

LGTM