If you install tobac from scratch on Python <3.10, import tobac fails with the following error:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/seanfreeman/mambaforge/envs/tobac_test_aux_geoir/lib/python3.9/site-packages/tobac/__init__.py", line 83, in <module>
    from . import merge_split
  File "/Users/seanfreeman/mambaforge/envs/tobac_test_aux_geoir/lib/python3.9/site-packages/tobac/merge_split.py", line 31, in <module>
    vertical_coord: Optional[str] | None = None,
TypeError: unsupported operand type(s) for |: '_UnionGenericAlias' and 'NoneType'

This makes sense, as the | character as an alias for union wasn't introduced until 3.10.

The fix for this is easy: add from __future__ import annotations to this file (and all other files), but this raises a key question: why did our matrix tests not catch this?

I think this fix warrants a v1.5.5, unfortunately.

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