Enable doctests for documentation snippets

[fayazkhan]

• Run doctest on all code snippets in the documentation.
  • This runs as part of the default nox sessions & Github Actions.
• Fix some of the incorrect documentation snippets.
• Add doctest for README.rst.
• Add doctest for tut_flask.rst.
• Add doctest_global_cleanup.
• Refactor testsetups into doctest_global_setup.
• Verify tests against the graph examples.
• Address warnings.

[coveralls]

coverage: 99.739%. remained the same
when pulling 860933c on update-docs
into da5b694 on master.