Move lint dependencies to extras

CONTRIBUTING.md

@@ -17,9 +17,9 @@ https://help.github.com/articles/using-pull-requests/) to ask us to pull your
 changes into our repo.
 
 Some other points to follow:
- 
+
  * Please base your changes on the `develop` branch.
-  
+
  * Please follow the [code style requirements](#code-style).
 
  * Please include a [changelog entry](#changelog) with each PR.
@@ -46,7 +46,7 @@ locally. You'll need python 3.6 or later, and to install a number of tools:
 
 ```
 # Install the dependencies
-pip install -U black flake8 flake8-comprehensions isort
+pip install -e .[lint]
 
 # Run the linter script
 ./scripts-dev/lint.sh

synapse/python_dependencies.py

@@ -104,6 +104,7 @@
     # hiredis is not a *strict* dependency, but it makes things much faster.
     # (if it is not installed, we fall back to slow code.)
     "redis": ["txredisapi>=1.4.7", "hiredis"],
+    "lint": ["isort==5.0.3", "black==19.10b0", "flake8-comprehensions", "flake8"],
 }
 
 ALL_OPTIONAL_REQUIREMENTS = set()  # type: Set[str]