Fix crash caused by typing.TypeVar lookup failure.

I previously fixed the pyi parser's handling of `from typing import TypeVar as
_TypeVar`, but I neglected to check that the parsed ast could then be
successfully resolved by load_pytd.

Context: python/typeshed#10201.
PiperOrigin-RevId: 535500076

pytype/load_pytd_test.py

@@ -637,6 +637,14 @@ class B: ...
     a = ast.Lookup("foo.A")
     self.assertEqual(a.Lookup("x").type.cls, a.Lookup("foo.A.B"))
 
+  def test_alias_typevar(self):
+    ast = self._import(foo="""
+      from typing import TypeVar as _TypeVar
+      T = _TypeVar('T')
+    """)
+    self.assertEqual(ast.Lookup("foo.T"),
+                     pytd.TypeParameter(name="T", scope="foo"))
+
 
 class ImportTypeMacroTest(_LoaderTest):
 

pytype/stubs/builtins/typing.pytd

@@ -1,5 +1,11 @@
-# Special-cased by the parser: TypeVar, Any, NamedTuple, Optional, Union
 __all__ = ...  # type: List[str]
+
+# Special-cased by the parser
+class Any: ...
+TypeVar: Any
+Optional: Any
+Union: Any
+
 if sys.version_info >= (3, 0):
   AnyStr = TypeVar('AnyStr', str, bytes)
 else: