Akuli and srittau: Remove Python 2 branches from Python 3 stubs (#5461)

* run script and do some manual changes (Akuli)

* do the whole thing manually (srittau)

* merge changes (Akuli)

Co-authored-by: Sebastian Rittau <srittau@rittau.biz>

stdlib/lib2to3/pytree.pyi

@@ -1,4 +1,3 @@
-import sys
 from lib2to3.pgen2.grammar import Grammar
 from typing import Any, Callable, Dict, Iterator, List, Optional, Text, Tuple, TypeVar, Union
 
@@ -36,9 +35,6 @@ class Base:
     def leaves(self) -> Iterator[Leaf]: ...
     def depth(self) -> int: ...
     def get_suffix(self) -> Text: ...
-    if sys.version_info < (3,):
-        def get_prefix(self) -> Text: ...
-        def set_prefix(self, prefix: Text) -> None: ...
 
 class Node(Base):
     fixers_applied: List[Any]