tests fail under python3.9: dummy_threading has been removed

[keszybz]

=================================== FAILURES ===================================
______ TestStandardLibraryReorganization.test_underscore_prefixed_modules ______

self = <test_future.test_standard_library.TestStandardLibraryReorganization testMethod=test_underscore_prefixed_modules>

    def test_underscore_prefixed_modules(self):
        import _thread
>       import _dummy_thread
E       ModuleNotFoundError: No module named '_dummy_thread'

tests/test_future/test_standard_library.py:427: ModuleNotFoundError
______________________ TestFutureMoves.test_future_moves _______________________

self = <test_future.test_standard_library.TestFutureMoves testMethod=test_future_moves>

    def test_future_moves(self):
        """
        Ensure everything is available from the future.moves interface that we
        claim and expect. (Issue #104).
        """
        from future.moves.collections import Counter, OrderedDict   # backported to Py2.6
        from future.moves.collections import UserDict, UserList, UserString
    
        from future.moves import configparser
        from future.moves import copyreg
    
        from future.moves.itertools import filterfalse, zip_longest
    
        from future.moves import html
        import future.moves.html.entities
        import future.moves.html.parser
    
        from future.moves import http
        import future.moves.http.client
        import future.moves.http.cookies
        import future.moves.http.cookiejar
        import future.moves.http.server
    
        from future.moves import queue
    
        from future.moves import socketserver
    
        from future.moves.subprocess import check_output              # even on Py2.6
        from future.moves.subprocess import getoutput, getstatusoutput
    
        from future.moves.sys import intern
    
        from future.moves import urllib
        import future.moves.urllib.error
        import future.moves.urllib.parse
        import future.moves.urllib.request
        import future.moves.urllib.response
        import future.moves.urllib.robotparser
    
        try:
            # Is _winreg available on Py2? If so, ensure future.moves._winreg is available too:
            import _winreg
        except ImportError:
            pass
        else:
            from future.moves import winreg
    
        from future.moves import xmlrpc
        import future.moves.xmlrpc.client
        import future.moves.xmlrpc.server
    
>       from future.moves import _dummy_thread

tests/test_future/test_standard_library.py:575: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

    from __future__ import absolute_import
    from future.utils import PY3
    
    if PY3:
>       from _dummy_thread import *
E       ModuleNotFoundError: No module named '_dummy_thread'

build/lib/future/moves/_dummy_thread.py:5: ModuleNotFoundError

See python/cpython@8bf08ee.

[sagitter]

Possible fix from Fedora's bug tracker.