Add flake8 pre-commit hook

Add flake8 to the pre-commit hooks. Also add `.flake8` configuration
file to the top-level project directory.

Closes #263.

.flake8

+[flake8]
+select = F,E101,E7,E9,W6
+extend-ignore = E741
+exclude = */options/*
\ No newline at end of file

.pre-commit-config.yaml

@@ -15,3 +15,7 @@ repos:
     rev: 5.12.0
     hooks:
     -   id: isort
+-   repo: https://github.com/pycqa/flake8
+    rev: 6.0.0
+    hooks:
+    -   id: flake8