externals: add astunparse

Add `astunparse` as `spack_astunparse`. This library unparses Python ASTs and we're
adding it under our own name so that we can make modifications to it.

Ultimately this will be used to make `package_hash` consistent across Python versions.

COPYRIGHT

@@ -34,6 +34,10 @@ includes the sbang tool directly in bin/sbang. These packages are covered
 by various permissive licenses. A summary listing follows. See the
 license included with each package for full details.
 
+PackageName: altgraph
+PackageHomePage: https://altgraph.readthedocs.io/en/latest/index.html
+PackageLicenseDeclared: MIT
+
 PackageName: argparse
 PackageHomePage: https://pypi.python.org/pypi/argparse
 PackageLicenseDeclared: Python-2.0
@@ -62,6 +66,10 @@ PackageName: jsonschema
 PackageHomePage: https://pypi.python.org/pypi/jsonschema
 PackageLicenseDeclared: MIT
 
+PackageName: macholib
+PackageHomePage: https://macholib.readthedocs.io/en/latest/index.html
+PackageLicenseDeclared: MIT
+
 PackageName: markupsafe
 PackageHomePage: https://pypi.python.org/pypi/MarkupSafe
 PackageLicenseDeclared: BSD-3-Clause
@@ -94,10 +102,6 @@ PackageName: six
 PackageHomePage: https://pypi.python.org/pypi/six
 PackageLicenseDeclared: MIT
 
-PackageName: macholib
-PackageHomePage: https://macholib.readthedocs.io/en/latest/index.html
-PackageLicenseDeclared: MIT
-
-PackageName: altgraph
-PackageHomePage: https://altgraph.readthedocs.io/en/latest/index.html
-PackageLicenseDeclared: MIT
+PackageName: spack_astunparse
+PackageHomePage: https://github.com/simonpercivall/astunparse
+PackageLicenseDeclared: Python-2.0