Remove version guard (#2565)

isAlive() is still present in threading.py in python 2.7, 3.4, 3.5, 3.6, 3.7.

Closes #2552

Author: PrajwalM2212

stdlib/2and3/threading.pyi

@@ -69,8 +69,7 @@ class Thread:
     def getName(self) -> str: ...
     def setName(self, name: str) -> None: ...
     def is_alive(self) -> bool: ...
-    if sys.version_info < (3,):
-        def isAlive(self) -> bool: ...
+    def isAlive(self) -> bool: ...
     def isDaemon(self) -> bool: ...
     def setDaemon(self, daemonic: bool) -> None: ...