gh-108455: Run mypy on Tools/peg_generator (#108456)

Co-authored-by: Hugo van Kemenade <hugovk@users.noreply.github.com>

.github/workflows/mypy.yml

@@ -9,6 +9,7 @@ on:
     paths:
       - "Tools/clinic/**"
       - "Tools/cases_generator/**"
+      - "Tools/peg_generator/**"
       - "Tools/requirements-dev.txt"
       - ".github/workflows/mypy.yml"
   workflow_dispatch:
@@ -29,7 +30,11 @@ jobs:
   mypy:
     strategy:
       matrix:
-        target: ["Tools/cases_generator", "Tools/clinic"]
+        target: [
+          "Tools/cases_generator",
+          "Tools/clinic",
+          "Tools/peg_generator",
+        ]
     name: Run mypy on ${{ matrix.target }}
     runs-on: ubuntu-latest
     timeout-minutes: 10