add immutability test to mixin

[atravitz]

It's my understanding that gufetokenizables should be immutable, and therefore this test should pass. open to discussion though.

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[github-actions]

No API break detected ✅

[IAlibay]

Our GufeTokenizables are unfortunately not all immutable, see SolventComponent:

In [1]: from gufe import SolventComponent

In [2]: a = SolventComponent()

In [3]: a.smiles
Out[3]: 'O'

In [4]: a._smiles = 'C'

In [5]: a.smiles
Out[5]: 'C'

The key is stable until you roundtrip on serialization.