Add flake8 annotions (#1502)

* Check for duplicates across dictionaries

And no longer allow self-corrections within these checks (given we exclude them elsewhere already)

* Add an annotating flake8 task

* Actually checkout the code in this step

* Add the missing newline

.github/workflows/codespell-private.yml

diff --git a/.github/workflows/codespell-private.yml b/.github/workflows/codespell-private.yml
index e7c5f00..2cb969f 100644
--- a/.github/workflows/codespell-private.yml
+++ b/.github/workflows/codespell-private.yml
@@ -9,3 +9,10 @@
     steps:
       - uses: actions/checkout@v2
       - run: make check-dictionaries
+
+  flake8-annotation:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - name: Flake8 with annotations
+        uses: TrueBrain/actions-flake8@v1.2