Replace pkg_resources version lookup

[Mirochill]

Description of the PR

Fixes #141.

This replaces the top-level pkg_resources version lookup with importlib.metadata, which is available in the standard library for the project's supported Python versions. The PackageNotFoundError fallback keeps the previous behavior for source trees that are not installed as a package.

The same replacement is applied to the remaining first-party pkg_resources usages in tests/simple_jump_process.py and notebooks/Stochasticity.ipynb, so the repository no longer imports pkg_resources directly.

Checks

This is an itemised checklist for the QA process within UKHSA and represents the bare minimum a QA should be.

To the reviewer: Check the boxes once you have completed the checks below.

• CI is successful
  • Did the test suite run sucessfully?

This is a basic form of Smoke Testing

• Data and security

  • Files containing individual user's secret files and config files are not in repo.
  • No private or identifiable data has been added to the repo.

• Sensible

  • Does the code execute the task accurately?
  • Is the code tidy, commented and parsimonious?
  • Does the code do what the comments and readme say it does*?
  • Is the code covered by useful unit tests?

• Documentation

  • The purpose of the code is clearly defined?
  • If reasonable, has an exaple of the code been given in a notebook in the docs?
  • Comments are included in the code so the reader can follow why the code behaves in the way it does
  • Is the code written in a standard way (does it pass linting)?
  • Variable, function & module names should be intuitive to the reader?

How this PR addresses the contribution requirements

• Justification: pkg_resources is no longer available with newer setuptools releases, and this import currently happens at package import time.
• Unit tests: no new unit test added; this is a direct replacement of the version metadata API and the fallback behavior is preserved.
• Documentation: no documentation prose changes; the existing Stochasticity.ipynb notebook import snippet was updated to avoid pkg_resources.
• CI: pending on this draft PR.
• Local validation:
  • git diff --check
  • git diff --cached --check
  • git diff --check HEAD~1..HEAD
  • rg -n pkg_resources|get_distribution|DistributionNotFound .
  • Not run locally