test: Eliminate flake8 errors in tests

[aucampia]

Summary of changes

Eliminate some occurrences of the following flake8 errors in tests:

• E265 block comment should start with '# '
• E266 too many leading '#' for block comment
• E402 module level import not at top of file
• E712 comparison to False should be 'if cond is False:' or 'if not cond:'
• E712 comparison to True should be 'if cond is True:' or 'if cond:'
• E722 do not use bare 'except'
• F401 ... imported but unused
• F403 ... used; unable to detect undefined names
• F405 ... may be undefined, or defined from star imports: ...
• F541 f-string is missing placeholders
• F841 local variable 'result' is assigned to but never used
• N806 variable 'TEST_DIR' in function should be lowercase

This is pursuant to eliminating
flakeheaven, as it no longer supports the latest version of flake8
[ref].

Checklist

• Checked that there aren't other open pull requests for
  the same change.
• Checked that all tests and type checking passes.
• Considered granting push permissions to the PR branch,
  so maintainers can fix minor issues and keep your PR up to date.

[coveralls]

Coverage: 90.84%. Remained the same when pulling 2f93018 on aucampia:iwana-20230412T2313-fix_test_flake8 into 4ea1436 on RDFLib:main.