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`)

lib/spack/spack/cmd/license.py

@@ -34,8 +34,8 @@
     r'^bin/spack$',
     r'^bin/spack-python$',
 
-    # all of spack core
-    r'^lib/spack/spack/.*\.py$',
+    # all of spack core except unparse
+    r'^lib/spack/spack/(?!util/unparse).*\.py$',
     r'^lib/spack/spack/.*\.sh$',
     r'^lib/spack/spack/.*\.lp$',
     r'^lib/spack/llnl/.*\.py$',

lib/spack/spack/util/unparse/LICENSE

@@ -0,0 +1,64 @@
+LICENSE
+=======
+
+Copyright (c) 2014, Simon Percivall
+All rights reserved.
+
+Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:
+
+* Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.
+
+* Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.
+
+* Neither the name of AST Unparser nor the names of its contributors may be used to endorse or promote products derived from this software without specific prior written permission.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+
+PYTHON SOFTWARE FOUNDATION LICENSE VERSION 2
+--------------------------------------------
+
+1. This LICENSE AGREEMENT is between the Python Software Foundation
+("PSF"), and the Individual or Organization ("Licensee") accessing and
+otherwise using this software ("Python") in source or binary form and
+its associated documentation.
+
+2. Subject to the terms and conditions of this License Agreement, PSF hereby
+grants Licensee a nonexclusive, royalty-free, world-wide license to reproduce,
+analyze, test, perform and/or display publicly, prepare derivative works,
+distribute, and otherwise use Python alone or in any derivative version,
+provided, however, that PSF's License Agreement and PSF's notice of copyright,
+i.e., "Copyright (c) 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010,
+2011, 2012, 2013, 2014 Python Software Foundation; All Rights Reserved" are retained
+in Python alone or in any derivative version prepared by Licensee.
+
+3. In the event Licensee prepares a derivative work that is based on
+or incorporates Python or any part thereof, and wants to make
+the derivative work available to others as provided herein, then
+Licensee hereby agrees to include in any such work a brief summary of
+the changes made to Python.
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+4. PSF is making Python available to Licensee on an "AS IS"
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+DISCLAIMS ANY REPRESENTATION OR WARRANTY OF MERCHANTABILITY OR FITNESS
+FOR ANY PARTICULAR PURPOSE OR THAT THE USE OF PYTHON WILL NOT
+INFRINGE ANY THIRD PARTY RIGHTS.
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+5. PSF SHALL NOT BE LIABLE TO LICENSEE OR ANY OTHER USERS OF PYTHON
+FOR ANY INCIDENTAL, SPECIAL, OR CONSEQUENTIAL DAMAGES OR LOSS AS
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+Agreement.

lib/spack/spack/util/unparse/__init__.py

@@ -0,0 +1,19 @@
+# Copyright (c) 2014-2021, Simon Percivall and Spack Project Developers.
+#
+# SPDX-License-Identifier: Python-2.0
+# coding: utf-8
+
+from __future__ import absolute_import
+
+from six.moves import cStringIO
+
+from .unparser import Unparser
+
+__version__ = '1.6.3'
+
+
+def unparse(tree, py_ver_consistent=False):
+    v = cStringIO()
+    unparser = Unparser(py_ver_consistent=py_ver_consistent)
+    unparser.visit(tree, v)
+    return v.getvalue().strip() + "\n"