Use `frozenset` for `Specification.kTraitSet`

[foundrytom]

What

Define the (Python) Specifications trait set using a frozenset so it can be hashed.

Why

Simplifies switching on trait set in manager implementations in Python, where a handler map is a nice way to structure code.

ACs

• Generated Python Specifications use frozenset for their static kTraitSet

[feltech]

Re-opening due to the change being reverted in #94. This issue is blocked until we can bump the required version of pybind11 used by OpenAssetIO.