add pre-commit configuration

pre-commit (https://pre-commit.com/) is a tool that helps you easily
manage pre-commit hooks. We are using this largely for quality control
by running tools like flake8 and black.

.pre-commit-config.yaml

@@ -0,0 +1,30 @@
+repos:
+  - repo: https://github.com/pycqa/isort
+    rev: 5.8.0
+    hooks:
+      - id: isort
+        name: isort (python)
+      - id: isort
+        name: isort (cython)
+        types: [cython]
+      - id: isort
+        name: isort (pyi)
+        types: [pyi]
+  - repo: https://github.com/psf/black
+    rev: 20.8b1
+    hooks:
+      - id: black
+  - repo: https://gitlab.com/pycqa/flake8
+    rev: 3.9.0
+    hooks:
+      - id: flake8
+  - repo: https://github.com/pre-commit/pre-commit-hooks
+    rev: v2.3.0
+    hooks:
+      - id: check-added-large-files
+      - id: check-case-conflict
+      - id: check-json
+      - id: check-merge-conflict
+      - id: detect-private-key
+      - id: end-of-file-fixer
+      - id: trailing-whitespace