Added initial rework of the concurrent.futures module (#5646)

* Added initial rework of the concurrent.futures module

* Minor fixes

* Fixed some generics & changed to collections over typing for some types

* Switched thread to use queues instead of multiprocessing.queues

* More fixes

* More fixes following results from running tests locally

* Tmp commit of changes

* Minor flake8 fix

* Fixing some issues

* Fixed a weakref.ref issue

* Fixed one more weakref issue

* Fixed some issues with required version

* Fixed more python min version requirements

* More min version fixes

* Fixed misc error in workflow regarded outdated pip

* Replaced any usage of Optional and Union with proper form as described in the contributions guide

* Fixed issue with using Callable definition

* Fixed last seen issues as per review

* Fixed some basic issues & more proper import calls

* Update stdlib/concurrent/futures/process.pyi

Co-authored-by: Sebastian Rittau <srittau@rittau.biz>

* Update stdlib/concurrent/futures/process.pyi

Co-authored-by: Sebastian Rittau <srittau@rittau.biz>

* Minor fixes

* More minor fixes

* Fixed up some issues & cleaned up imports

* Removed usage of Union

* Changed wait method to use Set of Future to work with mypy-primer for Optuna repo

* Reverted change to wait method and DoneAndNotDoneFutures class

* Fixed DoneAndNotDoneFutures again

Co-authored-by: Akuli <akuviljanen17@gmail.com>
Co-authored-by: Sebastian Rittau <srittau@rittau.biz>

.github/workflows/tests.yml

@@ -121,6 +121,8 @@ jobs:
         uses: actions/setup-python@v2
         with:
           python-version: ${{ matrix.python-version }}
+      - name: Update pip
+        run: python -m pip install -U pip
       - name: Install dependencies
         run: pip install $(grep mypy== requirements-tests-py3.txt)
       - name: Run stubtest