Update slow tests marks and add script to automate

[matt-graham]

Fixes #1145

Adds a new script src/scripts/automation/mark_slow_tests.py which uses a JSON report of Pytest results generated using pytest-json-report plugin to identify slow running tests and update the decorators used to mark the tests in the source code. The script uses RedBaron to parse the modules' source code in to a lossless abstract source tree, and adds or removes the slow mark decorators to the relevant test functions identified from the test report. To avoid too much switching back and forth for tests with duration close to the threshold used to determine if a test is 'slow' (currently ~10s), a basic form of hysteresis is used with slow marks only added if measured test duration is above a upper threshold (default 11s) and only removed if test duration is below a slower threshold (default 9s). A tox environment which install additional dependencies (pytest-json-report and redbaron), runs pytest with JSON report output and then runs mark_slow_tests.py script to update the tests is also added.

Updates to the slow marks based on a local run of the tests and script are also included in this PR - we could possibly separate this out in to a different PR. It would probably also be worth exploring automating running the script as an Actions workflow, potentially doing something like running on a weekly schedule and opening a PR for any changes to slow marks.

[matt-graham]

Check failing due to not removing pytest import on modules for which all slow marks have been removed and no other pytest usages.

• Add check if there are any remaining pytest usages after removing slow marks and remove pytest import if so

[matt-graham]

In the interests of reviving this PR and keep the changes more minimal and so easier to review, I've remove the commits which ran this script to update the existing slow marks in the tests so this PR now only adds the script for automatically updating slow marks plus corresponding tox environment. We can look at actually applying to update the slow tests marks in a separate PR or series of PRs.