'ForkAwareLocal' object has no attribute 'connection' for multithreaded cp

When running gsutil -m cp -r gs://example/ ./ on a fairly large folder on macOS with the system Python 3.7 (to prevent the issues from #961), I see many instances of the following error:

Exception in thread Thread-4:
Traceback (most recent call last):
  File "/Applications/Xcode.app/Contents/Developer/Library/Frameworks/Python3.framework/Versions/3.7/lib/python3.7/multiprocessing/managers.py", line 788, in _callmethod
    conn = self._tls.connection
AttributeError: 'ForkAwareLocal' object has no attribute 'connection'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/Applications/Xcode.app/Contents/Developer/Library/Frameworks/Python3.framework/Versions/3.7/lib/python3.7/threading.py", line 917, in _bootstrap_inner
    self.run()
  File "/usr/local/Caskroom/google-cloud-sdk/latest/google-cloud-sdk/platform/gsutil/gslib/command.py", line 2348, in run
    cls = copy.copy(class_map[caller_id])
  File "<string>", line 2, in __getitem__
  File "/Applications/Xcode.app/Contents/Developer/Library/Frameworks/Python3.framework/Versions/3.7/lib/python3.7/multiprocessing/managers.py", line 792, in _callmethod
    self._connect()
  File "/Applications/Xcode.app/Contents/Developer/Library/Frameworks/Python3.framework/Versions/3.7/lib/python3.7/multiprocessing/managers.py", line 779, in _connect
    conn = self._Client(self._token.address, authkey=self._authkey)
  File "/Applications/Xcode.app/Contents/Developer/Library/Frameworks/Python3.framework/Versions/3.7/lib/python3.7/multiprocessing/connection.py", line 492, in Client
    c = SocketClient(address)
  File "/Applications/Xcode.app/Contents/Developer/Library/Frameworks/Python3.framework/Versions/3.7/lib/python3.7/multiprocessing/connection.py", line 619, in SocketClient
    s.connect(address)
ConnectionRefusedError: [Errno 61] Connection refused

As a result the command seems to hang after starting a first batch of downloads.