[3.7] bpo-27639: Correct return type for UserList slicing operation (GH-13169)

[miss-islington]

• BPO-27639: Correct return type for UserList slicing operation

Added logic to getitem magic method for UserList to ensure that the return
type matches that of self.
(cherry picked from commit b1c3167)

Co-authored-by: Michael Blahay [email protected]

https://bugs.python.org/issue27639

[miss-islington]

@mblahay and @markshannon: Status check is done, and it's a failure ❌ .

[mblahay]

The error during AppVeyor build does not seem to have anything to do with the change. Can the test be rerun.

[mblahay]

My local test execution on a version prior to this change is still showing the error, this time in Linux

0:01:38 load avg: 1.79 [ 25/416] test_asyncio passed (58 sec 429 ms)
Future exception was never retrieved
future:
Traceback (most recent call last):
File "/home/mb18433/Projects/cpython/cpython/Lib/asyncio/subprocess.py", line 152, in _feed_stdin
await self.stdin.drain()
File "/home/mb18433/Projects/cpython/cpython/Lib/asyncio/streams.py", line 353, in drain
await self._protocol._drain_helper()
File "/home/mb18433/Projects/cpython/cpython/Lib/asyncio/streams.py", line 209, in _drain_helper
await waiter
BrokenPipeError
Future exception was never retrieved
future:
Traceback (most recent call last):
File "/home/mb18433/Projects/cpython/cpython/Lib/asyncio/subprocess.py", line 152, in _feed_stdin
await self.stdin.drain()
File "/home/mb18433/Projects/cpython/cpython/Lib/asyncio/streams.py", line 353, in drain
await self._protocol._drain_helper()
File "/home/mb18433/Projects/cpython/cpython/Lib/asyncio/streams.py", line 209, in _drain_helper
await waiter
BrokenPipeError

[vstinner]

My local test execution on a version prior to this change is still showing the error, this time in Linux

There are warnings, but errors: test_asyncio passed (success). You may open an issue at https://bugs.python.org/ about these warnings.