Update black, and run auto formatting over the codebase (#9381)

- Update black version to the latest
 - Run black auto formatting over the codebase
    - Run autoformatting according to [`docs/code_style.md
`](https://github.com/matrix-org/synapse/blob/80d6dc9783aa80886a133756028984dbf8920168/docs/code_style.md)
 - Update `code_style.md` docs around installing black to use the correct version

docs/code_style.md

@@ -8,16 +8,16 @@ errors in code.
 
 The necessary tools are detailed below.
 
+First install them with:
+
+    pip install -e ".[lint,mypy]"
+
 -   **black**
 
     The Synapse codebase uses [black](https://pypi.org/project/black/)
     as an opinionated code formatter, ensuring all comitted code is
     properly formatted.
 
-    First install `black` with:
-
-        pip install --upgrade black
-
     Have `black` auto-format your code (it shouldn't change any
     functionality) with:
 
@@ -28,10 +28,6 @@ The necessary tools are detailed below.
     `flake8` is a code checking tool. We require code to pass `flake8`
     before being merged into the codebase.
 
-    Install `flake8` with:
-
-        pip install --upgrade flake8 flake8-comprehensions
-
     Check all application and test code with:
 
         flake8 synapse tests
@@ -41,10 +37,6 @@ The necessary tools are detailed below.
     `isort` ensures imports are nicely formatted, and can suggest and
     auto-fix issues such as double-importing.
 
-    Install `isort` with:
-
-        pip install --upgrade isort
-
     Auto-fix imports with:
 
         isort -rc synapse tests