Fix matplotlib listedcolormap deprecated argument N

[SylviaWhittle]

---

Before submitting a Pull Request please check the following.

• Existing tests pass.
• Documentation has been updated and builds. Remember to update as required...
  • docs/usage/configuration.md
  • docs/usage/data_dictionary.md
  • docs/usage/advanced.md and new pages it should link to.
• Pre-commit checks pass.
• New functions/methods have typehints and docstrings.
• New functions/methods have tests which check the intended behaviour is correct.

Optional

topostats/default_config.yaml

If adding options to topostats/default_config.yaml please ensure.

• There is a comment adjacent to the option explaining what it is and the valid values.
• A check is made in topostats/validation.py to ensure entries are valid.
• Add the option to the relevant sub-parser in topostats/entry_point.py.

[SylviaWhittle]

The tests pass locally, and the test that fails here is a test_process_scan which should not be failing.

[SylviaWhittle]

The fix seems to work locally for me.

[SylviaWhittle]

Confirmed to work.

[SylviaWhittle]

Ubuntu 3.10 passes, this is floating point error. Forcing it through.