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.

Convert the remaining unittest based classes to pytest.

This patch primarily adds more type hints for `rdflib.graph`, but also
adds type hints to some related modules in order to work with the new
type hints for `rdflib.graph`.

I'm mainly doing this as a baseline for adding type hints to
`rdflib.store`.

I have created type aliases to make it easier to type everything
consistently and to make type hints easier easier to change in the
future. The type aliases are private however (i.e. `_`-prefixed) and
should be kept as such for now.

This patch only contains typing changes and does not change runtime
behavior.

Broken off from https://github.com/RDFLib/rdflib/pull/1850

`flake8` will eventually complain about this when the `flakeheaven` baseline
is invalidated.

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.