CI: drop Python 3.7 & add 3.10 for lama-to-dask workflow

[sadielbartholomew]

For context, see #313 (comment) and related comments in that thread. Also adding Python 3.10 whilst I am amending the workflow matrix, as that is now available and it is a good idea to start testing with it.

Trivial change but opening as a PR instead of committing directly to the branch in order to check the remaining jobs pass. (lama was changed to Llama somehow when writing the commit message here and it can stay for amusement value / because I can't be bothered to rebase to re-word).

[sadielbartholomew]

Ah, 3.10 is being interpreted as 3.1 naturally, let me try again...

[sadielbartholomew]

Retriggering via open-close...

[sadielbartholomew]

Oh, fair, the Python 3.10 jobs (empirically) won't pass because something about cfdm doesn't seem to play ball with a 3.10 update (yet to investigated):

Traceback (most recent call last):
  File "/Users/runner/work/cf-python/cf-python/main/cf/test/test_Data.py", line 23, in <module>
    import cf
  File "/Users/runner/work/cf-python/cf-python/main/cf/__init__.py", line 87, in <module>
    import cfdm
  File "/Users/runner/work/cf-python/cf-python/cfdm/cfdm/__init__.py", line 44, in <module>
    from . import core
  File "/Users/runner/work/cf-python/cf-python/cfdm/cfdm/core/__init__.py", line 60, in <module>
    from .domain import Domain
  File "/Users/runner/work/cf-python/cf-python/cfdm/cfdm/core/domain.py", line 4, in <module>
    class Domain(mixin.FieldDomain, abstract.Properties):
  File "/Users/runner/work/cf-python/cf-python/cfdm/cfdm/core/meta/docstringrewrite.py", line [20](https://github.com/NCAS-CMS/cf-python/runs/5130994352?check_suite_focus=true#step:13:20)5, in __new__
    wrapper.__wrapped__ = attr
AttributeError: readonly attribute
Error: Process completed with exit code 1.

so maybe that was a bit too optimistic. Removing 3.10 from the workflow, we'll just run on 3.8 and 3.9 for now...

[sadielbartholomew]

Should just be the 3.8 and 3.9 jobs now, retriggering...

[sadielbartholomew]

All good to merge now.

[davidhassell]

Thanks, Sadie.