:sparkles: Add dependencies with yield (used as context managers) (#595)

:sparkles: Add dependencies with yield (used as context managers) (#595)

* :heavy_plus_sign: Add development/testing dependencies for Python 3.6

* :sparkles: Add concurrency submodule with contextmanager_in_threadpool

* :sparkles: Add AsyncExitStack to ASGI scope in FastAPI app call

* :sparkles: Use async stack for contextmanager-able dependencies

including running in threadpool sync dependencies

* :white_check_mark: Add tests for contextmanager dependencies

including internal raise checks when exceptions should be handled and when not

* :white_check_mark: Add test for fake asynccontextmanager raiser

* :bug: Fix mypy errors and coverage

* :mute: Remove development logs and prints

* :white_check_mark: Add tests for sub-contextmanagers, background tasks, and sync functions

* :bug: Fix mypy errors for Python 3.7

* :speech_balloon: Fix error texts for clarity

* :memo: Add docs for dependencies with yield

* :sparkles: Update SQL with SQLAlchemy tutorial to use dependencies with yield

and add an alternative with a middleware (from the old tutorial)

* :white_check_mark: Update SQL tests to remove DB file during the same tests

* :white_check_mark: Add tests for example with middleware

as a copy from the tests with dependencies with yield, removing the DB in the tests

* :pencil2: Fix typos with suggestions from code review

Co-Authored-By: dmontagu <35119617+dmontagu@users.noreply.github.com>