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](https://github.com/flakeheaven/flakeheaven), as it no longer
supports the latest version of flake8
[[ref](https://github.com/flakeheaven/flakeheaven/issues/132)].

Disable
[`implicit_reexport`](https://mypy.readthedocs.io/en/stable/config_file.html#confval-implicit_reexport)
and eliminate all errors reported by mypy after this.

This helps ensure that import statements import from the right module and that
the `__all__` variable is correct.

This is being done because `rdlib.net` is a typo and `rdflib.net` is not owned or associated with this project.

Also:
- Expand testing for some parts of the code impacted by this.

Co-authored-by: Iwan Aucamp <aucampia@gmail.com>

This patch removes unused imports in test code. This is mainly to remove
the potential snag for flake8/flakeheaven.

The actual cleanup was done with `pycln --all test`.

Also:
 - Remove superflous `pass` statements.

for more information, see https://pre-commit.ci

Preparatory work for dataset-rework, eases review