Support gast.unparse

This allows to remove the test dependency on astunparse, while adapting
CPython's astunparse to gast AST

Fix #64

.github/workflows/core.yml

@@ -22,7 +22,7 @@ jobs:
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip
-        pip install astunparse pytest
+        pip install pytest
     - name: Setup
       run: |
         python setup.py install

gast/gast.py

@@ -307,6 +307,11 @@ def parse(*args, **kwargs):
     return ast_to_gast(_ast.parse(*args, **kwargs))
 
 
+def unparse(gast_obj):
+    from gast.unparser import unparse
+    return unparse(gast_obj)
+
+
 def literal_eval(node_or_string):
     if isinstance(node_or_string, AST):
         node_or_string = gast_to_ast(node_or_string)