[enh] #186: Introduce pyupgrade pre-commit hook

Document pyupgrade in contributing

.pre-commit-config.yaml

@@ -20,6 +20,11 @@ repos:
     rev: v2.8.2
     hooks:
     - id: pylint
+-   repo: https://github.com/asottile/pyupgrade
+    rev: v2.23.3
+    hooks:
+    - id: pyupgrade
+      args: [--py36-plus]
 -   repo: https://github.com/Lucas-C/pre-commit-hooks
     rev: v1.1.10
     hooks:

CONTRIBUTING.md

@@ -10,7 +10,7 @@ main <− ###_feature_branch
 
 ## Pre-commit
 We are using [`pre-commit`](https://pre-commit.com/) tool to perform checks on staged changes before committing.
-We are using it for `black` formatting, `isort` imports sorting, `pylint` code linting, `mypy` typing, and `gitlab-ci` linting.
+We are using it for `black` formatting, `isort` imports sorting, `pylint` code linting, `pyupgrade` syntax upgrader, `mypy` typing, and `gitlab-ci` linting.
 
 Install `pre-commit` from your distribution. In case it is an outdated version, install it from `pip`:
 ```bash