Add pre-commit plugin for black/isort/flake8 (#1398)

* Add pre-commit plugin for black/isort/flake8

* Update .pre-commit-config.yaml

* Add a few more useful hooks

.isort.cfg

@@ -4,5 +4,5 @@ include_trailing_comma=True
 force_grid_wrap=0
 use_parentheses=True
 line_length=88
-known_third_party=deprecated,httpretty,jwt,parameterized,requests,setuptools,six,urllib3
+known_third_party=deprecated,httpretty,jwt,parameterized,requests,setuptools,urllib3
 known_first_party=github

.pre-commit-config.yaml

@@ -0,0 +1,23 @@
+repos:
+  - repo: https://github.com/psf/black
+    rev: stable
+    hooks:
+      - id: black
+        language_version: python3.6
+  - repo: https://github.com/asottile/seed-isort-config
+    rev: v1.9.4
+    hooks:
+      - id: seed-isort-config
+  - repo: https://github.com/pre-commit/mirrors-isort
+    rev: v4.3.21
+    hooks:
+      - id: isort
+  - repo: https://gitlab.com/pycqa/flake8
+    rev: 3.7.9
+    hooks:
+      - id: flake8
+  - repo: https://github.com/pre-commit/pre-commit-hooks
+    rev: v2.3.0
+    hooks:
+      - id: end-of-file-fixer
+      - id: trailing-whitespace

CONTRIBUTING.md

@@ -23,7 +23,7 @@ Pull Requests should clearly describe two things:
 
 Ideally, changes should be made in logical commits and tests added to improve the project's coverage of the GitHub API.
 
-## Coding conventions
+## Coding style
 
 PyGithub adopts the black coding style and uses isort to sort imports.
 
@@ -32,6 +32,16 @@ To manually format the code:
 tox -e format
 ```
 
+## Pre-commit plugin
+
+To forget about coding style and let [pre-commit](https://pre-commit.com/#installation) fix your flake8/isort/black issue.
+
+```
+pre-commit install
+```
+
+That's it!
+
 ## Adding missing attributes for a GithubObject
 
 ```bash