Make appropriate ast methods generic (#2715)

Closes #2714

stdlib/3/ast.pyi

@@ -4,7 +4,7 @@
 # from _ast below when loaded in an unorthodox way by the Dropbox
 # internal Bazel integration.
 import typing as _typing
-from typing import Any, Iterator, Optional, Union
+from typing import Any, Iterator, Optional, Union, TypeVar
 
 from _ast import *
 
@@ -15,12 +15,14 @@ class NodeVisitor():
 class NodeTransformer(NodeVisitor):
     def generic_visit(self, node: AST) -> Optional[AST]: ...
 
+_T = TypeVar('_T', bound=AST)
+
 def parse(source: Union[str, bytes], filename: Union[str, bytes] = ..., mode: str = ...) -> Module: ...
-def copy_location(new_node: AST, old_node: AST) -> AST: ...
+def copy_location(new_node: _T, old_node: AST) -> _T: ...
 def dump(node: AST, annotate_fields: bool = ..., include_attributes: bool = ...) -> str: ...
-def fix_missing_locations(node: AST) -> AST: ...
+def fix_missing_locations(node: _T) -> _T: ...
 def get_docstring(node: AST, clean: bool = ...) -> str: ...
-def increment_lineno(node: AST, n: int = ...) -> AST: ...
+def increment_lineno(node: _T, n: int = ...) -> _T: ...
 def iter_child_nodes(node: AST) -> Iterator[AST]: ...
 def iter_fields(node: AST) -> Iterator[_typing.Tuple[str, Any]]: ...
 def literal_eval(node_or_string: Union[str, AST]) -> Any: ...