Test regularly against dependencies

[pp-mo]

Because the tests are constructing an env from scratch each time they run,
we are currently relying on ongoing development activity to spot any problems that might arise when new dependency versions are released.

So for instance, we didn't quickly notice the failures when Iris 3.10 introduced problems for us
More worrying, we likewise won't immediately notice if xarray changes so as to break our interface to it (which does involves rather less stable assumptions about xarray)

So, we should introduce a regular-running check of ncdata tests against latest dependencies.

(and N.B. if dependency breakages ever become frequent and get in the way of development, we can follow Iris et al to a known way out : use lockfiles + regular lockfile updates
)