thirdparty: remove Python 2-specific selectors2 copy

The selectors module was added in Python 3.4. Because we require Python 3.6, it
will always be available. Therefore the selectors2 module is not imported.

I’ve verified that the selectors2-specific workaround in commandserver.py is not
necessary with the selectors module from the standard library. It returns an
empty list if timeout was exceeded.

The pytype directive was needed to silence the following error:

File "/tmp/mercurial-ci/mercurial/worker.py", line 299, in _posixworker: No attribute 'close' on int [attribute-error]
  In Union[_typeshed.HasFileno, int]
File "/tmp/mercurial-ci/mercurial/worker.py", line 299, in _posixworker: No attribute 'close' on _typeshed.HasFileno [attribute-error]
  In Union[_typeshed.HasFileno, int]

mercurial/commandserver.py

@@ -10,18 +10,12 b' import errno'
 import gc
 import os
 import random
+import selectors
 import signal
 import socket
 import struct
 import traceback
 
-try:
-    import selectors
-
-    selectors.BaseSelector
-except ImportError:
-    from .thirdparty import selectors2 as selectors
-
 from .i18n import _
 from .pycompat import getattr
 from . import (
@@ -644,15 +638,7 b' class unixforkingservice:'
                 # waiting for recv() will receive ECONNRESET.
                 self._unlinksocket()
                 exiting = True
-            try:
-                events = selector.select(timeout=h.pollinterval)
-            except OSError as inst:
-                # selectors2 raises ETIMEDOUT if timeout exceeded while
-                # handling signal interrupt. That's probably wrong, but
-                # we can easily get around it.
-                if inst.errno != errno.ETIMEDOUT:
-                    raise
-                events = []
+            events = selector.select(timeout=h.pollinterval)
             if not events:
                 # only exit if we completed all queued requests
                 if exiting:

mercurial/worker.py

@@ -9,18 +9,12 b''
 import errno
 import os
 import pickle
+import selectors
 import signal
 import sys
 import threading
 import time
 
-try:
-    import selectors
-
-    selectors.BaseSelector
-except ImportError:
-    from .thirdparty import selectors2 as selectors
-
 from .i18n import _
 from . import (
     encoding,
@@ -304,7 +298,9 b' def _posixworker(ui, func, staticargs, a'
                         yield res
                 except EOFError:
                     selector.unregister(key.fileobj)
+                    # pytype: disable=attribute-error
                     key.fileobj.close()
+                    # pytype: enable=attribute-error
                     openpipes -= 1
                 except IOError as e:
                     if e.errno == errno.EINTR: