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)].

More or less complete type hints for `rdflib.plugins.sparql.algebra` and
`rdflib.plugins.sparql.operators`.

This does not change runtime behaviour.

Other changes:
- Fixed line endings of `test/test_issues/test_issue1043.py`
  and `test/test_issues/test_issue910.py`.
- Removed a type hint comment that was present in rdflib/plugins/sparql/algebra.py

This is split-off from https://github.com/RDFLib/rdflib/pull/1850.

from attempting to access language and datatype attributes of non-Literal objects

Added `isinstance` check to avoid AttributeErrors being raised by attempting to access `language` and `datatype`attribs of non-Literal objects being passed back up the call stack resulting from SPARQL BIND failures/errors.

Remove the extra newline generated by the N-Triples serializer and add
a unit test to verify that N-Triples output does not end in two
consecutive newlines.

Closes #1998 

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

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.

This patch moves all test utility modules into `test/utils/` and the
tests for these modules into `test/utils/test/`.

This is to make test utilities more organized and in preparation for
adding more test utilities related to test suites.