setup.cfg/pre-commit-config: Add flake8 hook and config

Now that things are fixed up, add flake8. No fancy plugins, just
base flake8 for now.

.pre-commit-config.yaml

@@ -19,5 +19,9 @@ repos:
   hooks:
   - id: pyupgrade
     args: ["--py37-plus"]
+- repo: https://github.com/pycqa/flake8
+  rev: 5.0.4  # 6.0.0 requires Python 3.8
+  hooks:
+  - id: flake8
 
 exclude: doc/ext/sphinxarg

setup.cfg

@@ -1,2 +1,13 @@
 [bdist_wheel]
 universal = 1
+
+[flake8]
+max_line_length = 100
+# E402: module level import not at top of file
+# E203: whitespace before ':' (black / flake8 disagreement)
+# W503: line break before binary operator (black / flake8 disagreement)
+ignore=E203,W503
+# E402: module level import not at top of file
+# F405: may be undefined, or defined from star imports
+# F403: from .manager import *' used; unable to detect undefined names
+per-file-ignores = */__init__.py: F405,F403,E402