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/_random.pyi

@@ -1,4 +1,3 @@
-import sys
 from typing import Tuple
 
 # Actually Tuple[(int,) * 625]
@@ -11,5 +10,3 @@ class Random(object):
     def setstate(self, __state: _State) -> None: ...
     def random(self) -> float: ...
     def getrandbits(self, __k: int) -> int: ...
-    if sys.version_info < (3,):
-        def jumpahead(self, i: int) -> None: ...