fix: correct no-prefix no-suffix exclude for top-level dirs (#975)

[b-kamphorst]

Bandit 1.7.5 has some inconsistent behaviour. Assume a directory with just one subdirectory in it, which in turn contains a single file: ./tests/build.py

The following behaviour is observed:

• bandit . -vrx ./tests ignores the file,
• bandit . -vrx test ignores the file,
• bandit . -vrx tests does NOT ignore the file.

This PR removes the internal tweaking of an excluded path if the path is a directory.

No breaking change is foreseen.

Closes #975

[b-kamphorst]

Friendly reminder to @ericwb, @lukehinds and/ or @sigmavirus24 for considering this PR.

[b-kamphorst]

Another friendly reminder to @ericwb, @lukehinds and/ or @sigmavirus24. Your consideration of this PR would be much appreciated :)