Python: Add parser support for lazy imports

As defined in PEP-810. We implement this in much the same way as how we
handle `async` annotations currently. The relevant nodes get an
`is_lazy` field that defaults to being false.

python/extractor/semmle/python/ast.py

@@ -845,17 +845,19 @@ class If(stmt):
 
 
 class Import(stmt):
-    __slots__ = "names",
+    __slots__ = "is_lazy", "names",
 
-    def __init__(self, names):
+    def __init__(self, names, is_lazy=False):
         self.names = names
+        self.is_lazy = is_lazy
 
 
 class ImportFrom(stmt):
-    __slots__ = "module",
+    __slots__ = "is_lazy", "module",
 
-    def __init__(self, module):
+    def __init__(self, module, is_lazy=False):
         self.module = module
+        self.is_lazy = is_lazy
 
 
 class Nonlocal(stmt):

python/extractor/semmle/python/parser/dump_ast.py

@@ -72,8 +72,8 @@ class AstDumper(object):
             # just not print it in that case.
             if field == "parenthesised" and value is None:
                 continue
-            # Likewise, the default value for `is_async` is `False`, so we don't need to print it.
-            if field == "is_async" and value is False:
+            # Likewise, the default value for `is_async` and `is_lazy` is `False`, so we don't need to print it.
+            if field in ("is_async", "is_lazy") and value is False:
                 continue
             output.write("{}  {}:".format(indent,field))
             if isinstance(value, list):

python/extractor/semmle/python/parser/tsg_parser.py

@@ -291,7 +291,7 @@ def create_placeholder_args(cls):
     if cls in (ast.Raise, ast.Ellipsis):
         return {}
     fields = ast_fields[cls]
-    args = {field: None for field in fields if field != "is_async"}
+    args = {field: None for field in fields if field not in ("is_async", "is_lazy")}
     for field in list_fields.get(cls, ()):
         args[field] = []
     if cls in (ast.GeneratorExp, ast.ListComp, ast.SetComp, ast.DictComp):