Remove --python-version 3.7 references from tests (#16848)

Remove `--python-version 3.7` from tests if not necessary anymore or use
`3.8` where it makes sense instead.

Author: cdce8p

test-data/unit/check-async-await.test

@@ -819,8 +819,6 @@ def bar() -> None:
 [typing fixtures/typing-async.pyi]
 
 [case testAsyncForOutsideCoroutine]
-# flags: --python-version 3.7
-
 async def g():
     yield 0
 
@@ -841,8 +839,6 @@ async for x in g(): ... # E: "async for" outside async function
 [typing fixtures/typing-async.pyi]
 
 [case testAsyncWithOutsideCoroutine]
-# flags: --python-version 3.7
-
 class C:
     async def __aenter__(self): pass
     async def __aexit__(self, x, y, z): pass
@@ -858,7 +854,6 @@ async with C() as x: # E: "async with" outside async function
 [typing fixtures/typing-async.pyi]
 
 [case testAwaitMissingNote]
-# flags: --python-version 3.7
 from typing import Generic, TypeVar, Generator, Any, Awaitable, Type
 
 class C:

test-data/unit/check-callable.test

@@ -525,7 +525,6 @@ else:
 [builtins fixtures/callable.pyi]
 
 [case testBuiltinsTypeAsCallable]
-# flags: --python-version 3.7
 from __future__ import annotations
 
 reveal_type(type)  # N: Revealed type is "def (x: Any) -> builtins.type"

test-data/unit/check-dataclass-transform.test

@@ -22,7 +22,6 @@ Person('Jonh', 21, None)  # E: Too many arguments for "Person"
 [builtins fixtures/dataclasses.pyi]
 
 [case testDataclassTransformIsFoundInTypingExtensions]
-# flags: --python-version 3.7
 from typing import Type
 from typing_extensions import dataclass_transform