bpo-40050: Fix importlib._bootstrap_external

[vstinner]

Remove two unused imports: _thread and _weakref. Avoid creating a new
winreg builtin module if it's already available in sys.modules.

The winreg module is now stored as "winreg" rather than "_winreg".

https://bugs.python.org/issue40050

[vstinner]

I tested manually that test_importlib no longer leaks with this PR if I reapply commit 8334f30:

$ ./python -m test -R 3:3 test_importlib 
(...)
beginning 6 repetitions
123456
......
(...)
Tests result: SUCCESS

$ git log

commit b7acc84ec0ba3dd092b0a5f879559fc1df0b5d7b (HEAD -> importlib_builtins)
Author: Hai Shi <shihai1992@gmail.com>
Date:   Fri Mar 20 16:16:45 2020 +0800

    [bpo-1635741](https://bugs.python.org/issue1635741): Port _weakref extension module to multiphase initialization (PEP 489) (GH-19084)

commit ee7422bfa01164e4077140333eabe663b6aab5e9 (origin/importlib_builtins)
Author: Victor Stinner <vstinner@python.org>
Date:   Tue Mar 24 15:54:39 2020 +0100

    [bpo-40050](https://bugs.python.org/issue40050): Fix importlib._bootstrap_external
    
    Remove two unused imports: _thread and _weakref. Avoid creating a new
    winreg builtin module if it's already available in sys.modules.
    
    The winreg module is now stored as "winreg" rather than "_winreg".

(...)