unparser: implement operator precedence algorithm for unparser

Backport operator precedence algorithm from here:
    397b96f6d7

This eliminates unnecessary parentheses from our unparsed output and makes Spack's unparser
consistent with the one in upstream Python 3.9+, with one exception.

Our parser normalizes argument order when `py_ver_consistent` is set, so that star arguments
in function calls come last.  We have to do this because Python 2's AST doesn't have information
about their actual order.

If we ever support only Python 3.9 and higher, we can easily switch over to `ast.unparse`, as
the unparsing is consistent except for this detail (modulo future changes to `ast.unparse`)

COPYRIGHT

@@ -42,6 +42,10 @@ PackageName: argparse
 PackageHomePage: https://pypi.python.org/pypi/argparse
 PackageLicenseDeclared: Python-2.0
 
+PackageName: astunparse
+PackageHomePage: https://github.com/simonpercivall/astunparse
+PackageLicenseDeclared: Python-2.0
+
 PackageName: attrs
 PackageHomePage: https://github.com/python-attrs/attrs
 PackageLicenseDeclared: MIT
@@ -101,7 +105,3 @@ PackageLicenseDeclared: Apache-2.0 OR MIT
 PackageName: six
 PackageHomePage: https://pypi.python.org/pypi/six
 PackageLicenseDeclared: MIT
-
-PackageName: spack_astunparse
-PackageHomePage: https://github.com/simonpercivall/astunparse
-PackageLicenseDeclared: Python-2.0