docs: document available security measures

Several security measures can be used to mitigate risk when processing
potentially malicious input.

This change adds documentation about available security measures and
examples and tests that illustrate their usage.

I want to add examples for securing RDFLib network access using
`sys.addaudithook` and `urllib.request.install_opener`, but I want to
also validate the examples in our CI pipeline, so we can demonstrate
they work to our users.

This change adds validation for all examples, and the addition of the
security examples in a seperate PR will then also get validated.

Enable nitpicky mode for Sphinx and fix all warnings and errors
that occur when running with nitpicky enabled.

Other changes:
- Add a tox environment for building docs (-docs). This is so we can
  test building docs on various versions of python as there seems to be
  some differences in warnings between different versions. This tox
  environment is enabled for linux CI builds.
- Change readthedocs to use python 3.9 as earlier versions do not handle
  `@typing.overload` with type aliases.
- Fixes https://github.com/RDFLib/rdflib/issues/1878

Updated two instances of lingering deprecated `tempfile.mktemp` usage to use current safe tempfile idiom.

* add: isort configure file

* fix: isort

$ isort .

* add: isort to dev deps

* add: isort to CI

* fix: move .isort.cfg into setup.cfg

* fix: re-formatted

* fix: isort target path

* Use pre-commit to check isort

pre-commit CI can auto fix this, and this way we can idenpendently evaluate
the formatting of the code from the validity of the code.

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

Removes the need to add quotation marks in the user input which otherwise causes the program to loop at  line 135 or crash at line 141.