Make sure version of black, isort and flake8 are the same across configs.

Author: matthias_schaub

.travis.yml

@@ -18,7 +18,7 @@ before_install:
 
 install:
   - pip install --upgrade pip setuptools
-  - pip install black flake8 isort
+  - pip install black==19.10b0 flake8==3.8.3 isort==5.5.2
 
 script:
   - black --check mapswipe_workers

docs/source/contributing.md

@@ -30,4 +30,7 @@ Use [pre-commit](https://pre-commit.com/) to run `black` and `flake8` prior to a
 pre-commit install
 ```
 
-From now on `black` and `flake8` should run automatically whenever `git commit` is executed.
+From now on `black`, `flake8` and `isort` should run automatically whenever `git commit` is executed.
+
+When running those tools manually please make sure the right version is used. The version can be looked up in `.pre-commit-config.yaml`.
+To update to newer version please make sure to change version numbers in `.pre-commit-config.yaml`, `.travis.yml` and `requirements.txt`.