Don't run actions twice on PRs

See https://github.community/t/how-to-trigger-an-action-on-push-or-pull-request-but-not-both/16662

.github/workflows/black.yaml

@@ -1,11 +1,15 @@
 name: black
 on:
   push:
+    branches:
+    - main
     paths:
     - '**.py'
     - 'pyproject.toml'
     - '.github/workflows/black.yaml'
   pull_request:
+    branches:
+    - main
     paths:
     - '**.py'
     - 'pyproject.toml'

.github/workflows/flake8.yaml

@@ -1,11 +1,15 @@
 name: flake8
 on:
   push:
+    branches:
+    - main
     paths:
     - '**.py'
     - '.flake8'
     - '.github/workflows/flake8.yaml'
   pull_request:
+    branches:
+    - main
     paths:
     - '**.py'
     - '.flake8'

.github/workflows/mypy.yaml

@@ -1,12 +1,16 @@
 name: mypy
 on:
   push:
+    branches:
+    - main
     paths:
     - '**.py'
     - 'mypy.ini'
     - 'requirements.txt'
     - '.github/workflows/mypy.yaml'
   pull_request:
+    branches:
+    - main
     paths:
     - '**.py'
     - 'mypy.ini'