Improve asyncio.tasks stubs (#2139)

- Use PEP 526-style variable annotations where possible
- Use FrameType instead of Any
- Remove workaround for mypy issue that was since fixed

stdlib/3.4/asyncio/tasks.pyi

@@ -1,6 +1,7 @@
 from typing import (Any, TypeVar, Set, Dict, List, TextIO, Union, Tuple, Generic, Callable,
                     Coroutine, Generator, Iterable, Awaitable, overload, Sequence, Iterator,
                     Optional)
+from types import FrameType
 import concurrent.futures
 from .events import AbstractEventLoop
 from .futures import Future
@@ -15,9 +16,9 @@ _T4 = TypeVar('_T4')
 _T5 = TypeVar('_T5')
 _FutureT = Union[Future[_T], Generator[Any, None, _T], Awaitable[_T]]
 
-FIRST_EXCEPTION = 'FIRST_EXCEPTION'
-FIRST_COMPLETED = 'FIRST_COMPLETED'
-ALL_COMPLETED = 'ALL_COMPLETED'
+FIRST_EXCEPTION: str
+FIRST_COMPLETED: str
+ALL_COMPLETED: str
 
 def as_completed(fs: Sequence[_FutureT[_T]], *, loop: AbstractEventLoop = ...,
                  timeout: Optional[float] = ...) -> Iterator[Generator[Any, None, _T]]: ...
@@ -57,22 +58,13 @@ def wait_for(fut: _FutureT[_T], timeout: Optional[float],
              *, loop: AbstractEventLoop = ...) -> Future[_T]: ...
 
 class Task(Future[_T], Generic[_T]):
-    _all_tasks = ...  # type: Set[Task]
-    _current_tasks = ...  # type: Dict[AbstractEventLoop, Task]
     @classmethod
     def current_task(cls, loop: AbstractEventLoop = ...) -> Task: ...
     @classmethod
     def all_tasks(cls, loop: AbstractEventLoop = ...) -> Set[Task]: ...
-
-    # Can't use a union, see mypy issue  #1873.
-    @overload
-    def __init__(self, coro: Generator[Any, None, _T],
-                 *, loop: AbstractEventLoop = ...) -> None: ...
-    @overload
-    def __init__(self, coro: Awaitable[_T], *, loop: AbstractEventLoop = ...) -> None: ...
-
+    def __init__(self, coro: Union[Generator[Any, None, _T], Awaitable[_T]], *, loop: AbstractEventLoop = ...) -> None: ...
     def __repr__(self) -> str: ...
-    def get_stack(self, *, limit: int = ...) -> List[Any]: ...  # return List[stackframe]
+    def get_stack(self, *, limit: int = ...) -> List[FrameType]: ...
     def print_stack(self, *, limit: int = ..., file: TextIO = ...) -> None: ...
     def cancel(self) -> bool: ...
     def _step(self, value: Any = ..., exc: Exception = ...) -> None: ...