unparser: drop Python 2, fix testing bugs with newer Pythons (#38424)

The `unparser` that Spack uses for package hashing had several tweaks to ensure compatibility
with Python 2.7:
1. Currently, the unparser automatically moves `*` and `**` args to the end to preserve
   compatibility with `python@:3.4`
2. `print a, b, c` statements and single-tuple `print((a, b, c))` function calls were
   remapped to `print(a, b, c)` in the unparsed output for consistency across versions.

(1) is causing issues in our tests because a recent patch to the Python source code  
(https://github.com/python/cpython/pull/102953/files#diff-7972dffec6674d5f09410c71766ac6caacb95b9bccbf032061806ae304519c9bR813-R823) 
has a `**` arg before an named argument, and we round-trip the core python source code
as a test of our unparser.  This isn't actually a break with our consistent unpausing -- it's still
consistent, the python source just doesn't unparse to the same thing anymore. It does makes
it harder to test, so it's not worth maintaining the Python2-specific stuff anymore.

Since we only support `python@3.6:`, this PR removes (1) and (2) from the unparser, but keeps
one last tweak for unicode AST inconsistencies, as it's still needed for Python 3.5-3.7.

This fixes the CI error we've been seeing on `python@3.11.4` and `python@3.10.12`. Again, that
bug exists only in the test system and doesn't affect our canonical hashing of Python code.

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