bugfix: preserve dict order for `Spec.dag_hash()` in Python 2 (#31092)

Fix a bug introduced in #21720. `spack_json.dump()` calls `_strify()` on dictionaries to
convert `unicode` to `str`, but it constructs `dict` objects instead of
`collections.OrderedDict` objects, so in Python 2 (or earlier versions of 3) it can
scramble dictionary order.

This can cause hashes to differ between Python 2 and Python 3, or between Python 3.7
and earlier Python 3's.

- [x] use `OrderedDict` in `_strify`
- [x] add a regression test

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