Loky

- Avoid a NameError when calling the `exit` builtin on Windows when
loky is executed as part of a frozen Python binary. (290)

- Make it possible to automatically trace workers when profiling with
VizTracer (299).

- Fix a side-effect bug in the registration of custom reducers the loky
subclass of `cloudpickle.CloudPickler` with cloudpickle 1.6.0. (272).

- Fix support for Python 3.9 and test against python-nightly from now on
(250).

- Add a parameter to ``cpu_count``, ``only_physical_cores``, to return the
number of physical cores instead of the number of logical cores (271).

- Fix thread-safety issues when iterating over the list of processes
(Dictionary changed sized during iteration) (263).

- Internal refactoring: add private factory class method to
``_ReusablePoolExecutor`` to ease extensibility in joblib (253).

- Increase the residual memory increase threshold (100MB -> 300MB) used by
loky for memory leak detection (causing loky workers to be
shutdown/restarted), in order to reduce the amount of false positives (238).

- In Python 3.8, loky processes now inherit multiprocessing's
``resource_tracker`` created from their parent. As a consequence, no spurious
``resource_tracker`` warnings are emitted when loky workers manipulate
``shared_memory`` objects (242).
Note that loky still needs to use its own resource tracker instance to manage
resources that require the reference counting logic such as joblib temporary
memory mapped files for now.

- The ``resource_tracker`` now comes with built-in support for tracking files
in all OSes. In addition, Python processes can now signal they do not need a
shared resource anymore by using the
``resource_tracker.maybe_unlink(resource_name, resource_type)`` method. After
all processes having access to the said resource have called this method, the
``resource_tracker`` will proceed to unlink the resource. Previously, resource
unlinking by the ``resource_tracker`` was only done for leaked resources at
interpreter exit (228).
- Fix `shutdown(wait=False)` that was potentially leading to deadlocks and froze
interpreters (246).
- Backport `ExecutorManagerThread` from cpython to refactor
`_queue_management_thread` and ease maintenance (246).

- Copy the environment variables in the child process for ``LokyProcess``. Also
add a ``env`` argument in ``LokyProcess``, ``ProcessPoolExecutor`` and
``get_reusable_executor`` to over-write consistently some environment variable
in the child process. This allows setting env variables before loading any
module.
Note: this feature is unreliable on Windows with Python < 3.6. (217)

- Fix a bug making all loky workers crash on Windows for Python>3.7 when using
a virtual environment (216).

- Fix a bug of the ``resource_tracker`` that could create unlimited freeze on
Windows (212)