Default nosec_lines to dictionary instead of list

[f-fl0]

This PR provides a simple fix to the following issue I ran into when trying to run bandit on a python script via flake8 from https://github.com/nastra/flake8-bandit/tree/fix-issue-with-latest-bandit

Traceback (most recent call last):
  File "/usr/local/bin/flake8", line 8, in <module>
    sys.exit(main())
  File "/usr/local/lib/python3.8/site-packages/flake8/main/cli.py", line 22, in main
    app.run(argv)
  File "/usr/local/lib/python3.8/site-packages/flake8/main/application.py", line 375, in run
    self._run(argv)
  File "/usr/local/lib/python3.8/site-packages/flake8/main/application.py", line 364, in _run
    self.run_checks()
  File "/usr/local/lib/python3.8/site-packages/flake8/main/application.py", line 271, in run_checks
    self.file_checker_manager.run()
  File "/usr/local/lib/python3.8/site-packages/flake8/checker.py", line 311, in run
    self.run_serial()
  File "/usr/local/lib/python3.8/site-packages/flake8/checker.py", line 295, in run_serial
    checker.run_checks()
  File "/usr/local/lib/python3.8/site-packages/flake8/checker.py", line 597, in run_checks
    self.run_ast_checks()
  File "/usr/local/lib/python3.8/site-packages/flake8/checker.py", line 500, in run_ast_checks
    for (line_number, offset, text, _) in runner:
  File "/usr/local/lib/python3.8/site-packages/flake8_bandit.py", line 136, in run
    for warn in self._check_source():
  File "/usr/local/lib/python3.8/site-packages/flake8_bandit.py", line 118, in _check_source
    bnv.generic_visit(self.tree)
  File "/usr/local/lib/python3.8/site-packages/bandit/core/node_visitor.py", line 263, in generic_visit
    if self.pre_visit(item):
  File "/usr/local/lib/python3.8/site-packages/bandit/core/node_visitor.py", line 208, in pre_visit
    nosec_tests = self.nosec_lines.get(node.lineno)
AttributeError: 'list' object has no attribute 'get'

How to reproduce

1. Create sample.py with the following content:

def foo(a: str) -> List[str]:
    return a.split()


def function_with_try_except() -> None:
    try:
        foo("test")
    except Exception:
        pass

2. Build a docker environment with the specific flake8-bandit
   Content of Dockerfile

FROM python:3.8-slim

RUN python -m pip install --no-cache-dir --upgrade pip \
  && python -m pip install --no-cache-dir https://github.com/nastra/flake8-bandit/archive/dfba03252ab83d5db2180fe5522816b730a5a993.tar.gz

COPY sample.py sample.py

CMD ["flake8", "sample.py"]

Build with docker build -t flake8-bandit-bug .

4. Run the container to observe the reported error
   docker run --rm flake8-bandit-bug

[f-fl0]

No longer necessary as tylerwince#26 was merged.