pre-commit: Use Python 3 for everything

This lets us use e.g. f-strings. We also bump the version of the default
pre-commit lib and migrate to the upstream flake8 plugin, since the old
one is now deprecated.

Signed-off-by: Stephen Finucane <stephen@that.guru>

Author: stephenfin

.pre-commit-config.yaml

@@ -1,16 +1,22 @@
+---
+default_language_version:
+  # force all unspecified python hooks to run python3
+  python: python3
 repos:
 - repo: https://github.com/pre-commit/pre-commit-hooks
-  rev: v2.3.0
+  rev: v2.5.0
   hooks:
   - id: check-executables-have-shebangs
   - id: check-merge-conflict
   - id: check-yaml
   - id: end-of-file-fixer
     exclude: (.*\.mbox)|(.*\.css)|(.*\.json)|(.*\.js)|(.*\.svg)|(.*/mails/.*-subject\.txt)
-  - id: flake8
-    exclude: ^patchwork/migrations
   - id: trailing-whitespace
     exclude: (.*\.mbox)|(.*\.svg)
+- repo: https://gitlab.com/pycqa/flake8
+  rev: 3.7.9
+  hooks:
+  - id: flake8
 - repo: https://github.com/Lucas-C/pre-commit-hooks
   rev: v1.1.7
   hooks: