Allow specific __future__ imports in modules

We do want to allow certain from __future__ imports in modules that make
it easier to code compatible python2 and python3.  Note that
unicode_literals is specifically left out and should never be allowed.
Now that python-3.4+ allows u"" there's no good reason to use
unicode_literals.

Also switch tables in the validate_modules documentation to simple table format

test/sanity/validate-modules/main.py

@@ -225,6 +225,8 @@ class ModuleValidator(Validator):
         'setup.ps1'
     ))
 
+    WHITELIST_FUTURE_IMPORTS = frozenset(('absolute_import', 'division', 'print_function'))
+
     def __init__(self, path, analyze_arg_spec=False, base_branch=None, git_cache=None, reporter=None):
         super(ModuleValidator, self).__init__(reporter=reporter or Reporter())
 
@@ -558,6 +560,20 @@ class ModuleValidator(Validator):
 
         for child in self.ast.body:
             if isinstance(child, (ast.Import, ast.ImportFrom)):
+                if isinstance(child, ast.ImportFrom) and child.module == '__future__':
+                    # allowed from __future__ imports
+                    for future_import in child.names:
+                        if future_import.name not in self.WHITELIST_FUTURE_IMPORTS:
+                            self.reporter.error(
+                                path=self.object_path,
+                                code=209,
+                                msg=('Only the following from __future__ imports are allowed: %s'
+                                     % ', '.join(self.WHITELIST_FUTURE_IMPORTS)),
+                                line=child.lineno
+                            )
+                            break
+                    else:  # for-else.  If we didn't find a problem nad break out of the loop, then this is a legal import
+                        continue
                 import_lines.append(child.lineno)
                 if child.lineno < min_doc_line:
                     self.reporter.error(