Update dependencies

Mypy is still pinned at 0.812, and will be updated to 0.9xx after the
pydantic plugin is compatible.

```text
.venv ❯ poetry update
Updating dependencies
Resolving dependencies... (19.2s)

Writing lock file

Package operations: 0 installs, 29 updates, 0 removals

  • Updating zipp (3.5.0 -> 3.6.0)
  • Updating click (8.0.1 -> 8.0.3)
  • Updating distlib (0.3.2 -> 0.3.3)
  • Updating filelock (3.0.12 -> 3.3.0)
  • Updating ghp-import (2.0.1 -> 2.0.2)
  • Updating importlib-metadata (4.6.3 -> 4.8.1)
  • Updating jinja2 (3.0.1 -> 3.0.2)
  • Updating platformdirs (2.2.0 -> 2.4.0)
  • Updating pluggy (0.13.1 -> 1.0.0)
  • Updating typing-extensions (3.10.0.0 -> 3.10.0.2)
  • Updating watchdog (2.1.3 -> 2.1.6)
  • Updating certifi (2021.5.30 -> 2021.10.8)
  • Updating cfgv (3.3.0 -> 3.3.1)
  • Updating charset-normalizer (2.0.4 -> 2.0.6)
  • Updating coverage (5.5 -> 6.0.1)
  • Updating identify (2.2.11 -> 2.3.0)
  • Updating mkdocs-material-extensions (1.0.1 -> 1.0.3)
  • Updating pygments (2.9.0 -> 2.10.0)
  • Updating pymdown-extensions (8.2 -> 9.0)
  • Updating pytest (6.2.4 -> 6.2.5)
  • Updating python-dotenv (0.19.0 -> 0.19.1)
  • Updating regex (2021.7.6 -> 2021.10.8)
  • Updating tomli (1.2.0 -> 1.2.1)
  • Updating urllib3 (1.26.6 -> 1.26.7)
  • Updating uvloop (0.15.3 -> 0.16.0)
  • Updating virtualenv (20.7.0 -> 20.8.1)
  • Updating websockets (9.1 -> 10.0)
  • Updating mkdocs-material (7.2.2 -> 7.3.3)
  • Updating pre-commit (2.13.0 -> 2.15.0)

.venv ❯ poetry add black@'21.9b0' --allow-prereleases --dev

Updating dependencies
Resolving dependencies... (0.7s)

Writing lock file

Package operations: 0 installs, 1 update, 1 removal

  • Removing appdirs (1.4.4)
  • Updating black (21.7b0 -> 21.9b0)

.venv ❯ poetry add flake8@'^4' --dev

Updating dependencies
Resolving dependencies... (2.3s)

Writing lock file

Package operations: 0 installs, 3 updates, 0 removals

  • Updating pycodestyle (2.7.0 -> 2.8.0)
  • Updating pyflakes (2.3.1 -> 2.4.0)
  • Updating flake8 (3.9.2 -> 4.0.1)

.venv ❯ pre-commit autoupdate
Updating https://github.com/psf/black ... updating 21.7b0 -> 21.9b0.
Updating https://github.com/pycqa/flake8 ... updating 3.9.2 -> 4.0.1.
Updating https://github.com/pycqa/isort ... already up to date.
Updating https://github.com/pre-commit/mirrors-prettier ... updating v2.3.2 -> v2.4.1.
Updating https://github.com/pre-commit/pre-commit-hooks ... already up to date.
```

.pre-commit-config.yaml

@@ -2,11 +2,11 @@ minimum_pre_commit_version: 2.8.0
 default_stages: [commit, push, manual]
 repos:
   - repo: https://github.com/psf/black
-    rev: 21.7b0
+    rev: 21.9b0
     hooks:
       - id: black
   - repo: https://github.com/pycqa/flake8
-    rev: 3.9.2
+    rev: 4.0.1
     hooks:
       - id: flake8
         args: [--max-line-length=88]
@@ -21,7 +21,7 @@ repos:
       - id: mypy
         language: system
   - repo: https://github.com/pre-commit/mirrors-prettier
-    rev: v2.3.2
+    rev: v2.4.1
     hooks:
       - id: prettier
   - repo: https://github.com/pre-commit/pre-commit-hooks