[pre-commit.ci] pre-commit autoupdate

updates:
- [github.com/asottile/pyupgrade: v2.13.0 → v2.14.0](https://github.com/asottile/pyupgrade/compare/v2.13.0...v2.14.0)
- [github.com/psf/black: 21.4b0 → 21.4b2](https://github.com/psf/black/compare/21.4b0...21.4b2)

release version 2.0.0rc2

Async improvements

It is better to encourage users to utilise the app ensure_sync method
(or the newely added async_to_sync method) so that any extensions that
alter these methods take affect throughout the users code.

With the helper method users code fix parts of their code to the
asgiref async_to_sync ignoring any extension changes.

Firstly `run_sync` was a misleading name as it didn't run anything,
instead I think `async_to_sync` is much clearer as it converts a
coroutine function to a function. (Name stolen from asgiref).

Secondly trying to run the ensure_sync during registration made the
code more complex and brittle, e.g. the _flask_async_wrapper
usage. This was done to pay any setup costs during registration rather
than runtime, however this only saved a iscoroutne check. It allows
the weirdness of the Blueprint and Scaffold ensure_sync methods to be
removed.

Switching to runtime ensure_sync usage provides a method for
extensions to also support async, as now documented.

This was required with the previous implementation of Werkzeug's
locals which didn't persist across threads. However as the current
implementation uses ContextVars which do persist the context copying
is no longer required.

Update changelog for #3932

Bump python-dotenv from 0.17.0 to 0.17.1

Bumps [python-dotenv](https://github.com/theskumar/python-dotenv) from 0.17.0 to 0.17.1.
- [Release notes](https://github.com/theskumar/python-dotenv/releases)
- [Changelog](https://github.com/theskumar/python-dotenv/blob/master/CHANGELOG.md)
- [Commits](https://github.com/theskumar/python-dotenv/compare/v0.17.0...v0.17.1)
Signed-off-by: Ndependabot[bot] <support@github.com>

Add tips for using async on Windows on Python 3.8

Bumps [pre-commit](https://github.com/pre-commit/pre-commit) from 2.12.0 to 2.12.1.
- [Release notes](https://github.com/pre-commit/pre-commit/releases)
- [Changelog](https://github.com/pre-commit/pre-commit/blob/master/CHANGELOG.md)
- [Commits](https://github.com/pre-commit/pre-commit/compare/v2.12.0...v2.12.1)
Signed-off-by: Ndependabot[bot] <support@github.com>

Upgrade to GitHub-native Dependabot

Allow using Click 7 with a DeprecationWarning

Move get_root_path back to flask.helpers

As long as popular libraries (e.g. Celery) require click 7, depending
on Click 8 in Flask makes it hard to test the latest version (and its
other dependencies) in existing applications.

Many libraries use it so the move is somewhat disruptive.

[pre-commit.ci] pre-commit autoupdate

updates:
- [github.com/asottile/pyupgrade: v2.12.0 → v2.13.0](https://github.com/asottile/pyupgrade/compare/v2.12.0...v2.13.0)
- [github.com/asottile/reorder_python_imports: v2.4.0 → v2.5.0](https://github.com/asottile/reorder_python_imports/compare/v2.4.0...v2.5.0)
- [github.com/psf/black: 20.8b1 → 21.4b0](https://github.com/psf/black/compare/20.8b1...21.4b0)

This should make it easier for users to correctly use Flask. The hints
are from Quart.

This enables type checking in CI and marks the project as typed.

This has been an early question from users, so best to explain.

Whilst it has been possible to serve via an ASGI server for a while
(using WSGI to ASGI middleware/adapters) it hasn't added much. Now
though it makes sense to recommend the asgiref adapter as it
integrates with the same event loop used for async route handlers
etc...

Release 2.0.0rc1