Add pytest-testmon pre-commit hook

[ns-rse]

We now use pytest-testmon as a pre-commit hook on
AFM-SPM/TopoStats as it helps us remember to make sure we don't break any
existing tests when working on the code base.

It would therefore be useful to have it enabled in AFMReader.

pyproject.toml

[project.optional-dependencies]
dev = [
  ...
  pytest-testmon,
  ...
]

.pre-commit-config.yaml

  - repo: local
    hooks:
      - id: pytest
        name: Pytest (testmon)
        entry: pytest --testmon
        language: system
        files: \.py$