Avoid iteration error when getting module path

[jrbourbeau]

While running Client.restart() as part of a Prefect workflow, I got the following error

12:47:35.902 | ERROR   | distributed.core - Exception while handling op restart
Traceback (most recent call last):
  File "/Users/james/projects/dask/distributed/distributed/utils.py", line 832, in wrapper
    return await func(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/james/projects/dask/distributed/distributed/scheduler.py", line 6292, in restart
    raise TimeoutError(msg) from None
TimeoutError: Waited for 1 worker(s) to reconnect after restarting, but after 120s, only 0 have returned. Consider a longer timeout, or `wait_for_workers=False`.

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/Users/james/projects/dask/distributed/distributed/core.py", line 970, in _handle_comm
    result = await result
             ^^^^^^^^^^^^
  File "/Users/james/projects/dask/distributed/distributed/utils.py", line 831, in wrapper
    with self:
  File "/Users/james/projects/dask/distributed/distributed/utils.py", line 853, in __exit__
    modname = _getmodulename_with_path(frame.filename)
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/james/projects/dask/distributed/distributed/utils.py", line 804, in _getmodulename_with_path
    for modname, mod in sys.modules.items():
RuntimeError: dictionary changed size during iteration

due to sys.modules changing.

This PR adds a .copy(), per the official Python docs https://docs.python.org/3/library/sys.html#sys.modules, to avoid this error.

[hendrikmakait]

Thanks, @jrbourbeau! LGTM, assuming CI ends up happy.

[github-actions]

Unit Test Results

See test report for an extended history of previous test failures. This is useful for diagnosing flaky tests.

    27 files  ±0      27 suites  ±0   9h 58m 59s ⏱️ - 5m 48s
 3 996 tests ±0   3 885 ✅ ±0    110 💤 ±0  1 ❌ ±0 
50 254 runs  ±0  47 957 ✅  - 1  2 296 💤 +1  1 ❌ ±0 

For more details on these failures, see this check.

Results for commit ffc3ff7. ± Comparison against base commit 390a5b5.