Added pre-commit (#752)

- This PR adds the checks that are defined in the Makefile as pre-commit
hooks.
- Hopefully, the checks are equivalent to those from the Makefile, but I
can't guarantee it.
- CI remains as it is.
- As I pointed out on discord, I experienced some conflicts between
flake8 and yapf, so it might be better to transition to some other
combination (e.g. black).

.pre-commit-config.yaml

@@ -0,0 +1,35 @@
+---
+repos:
+  # - repo: local
+  #   hooks:
+  #     - id: mypy
+  #       name: mypy
+  #       entry: mypy
+  #       language: python
+  #       pass_filenames: false
+  #       args: [--config-file=setup.cfg, tianshou]
+
+  - repo: https://github.com/pre-commit/mirrors-yapf
+    rev: v0.32.0
+    hooks:
+      - id: yapf
+        args: [-r]
+
+  - repo: https://github.com/pycqa/isort
+    rev: 5.10.1
+    hooks:
+      - id: isort
+        name: isort
+
+  - repo: https://gitlab.com/PyCQA/flake8
+    rev: 4.0.1
+    hooks:
+      - id: flake8
+        args: [--config=setup.cfg, --count, --show-source, --statistics]
+        additional_dependencies: ["flake8_bugbear"]
+
+  - repo: https://github.com/pycqa/pydocstyle
+    rev: 6.1.1
+    hooks:
+      - id: pydocstyle
+        exclude: ^(test/)|(docs/)|(examples/)|(setup.py)