Split Mypy checks into external and internal

Since the error codes change across versions, it's too complicated
(and kinda pointless) to test our code on all versions.

.github/workflows/ci.yml

@@ -110,8 +110,18 @@ jobs:
 
       - run: python -Im tox run -e system-argon2
 
-  mypy:
-    name: Mypy on ${{ matrix.python-version }}
+  mypy-all:
+    name: Mypy for API & own code
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+      - uses: actions/setup-python@v4
+
+      - run: python -Im pip install --upgrade wheel tox
+      - run: python -Im tox run -e mypy-api
+
+  mypy-api:
+    name: Mypy for API on ${{ matrix.python-version }}
     runs-on: ubuntu-latest
     strategy:
       fail-fast: false
@@ -132,7 +142,7 @@ jobs:
           allow-prereleases: true
 
       - run: python -Im pip install --upgrade wheel tox
-      - run: python -Im tox run -e mypy
+      - run: python -Im tox run -e mypy-api
 
   docs:
     name: Build docs & run doctests

tox.ini

@@ -2,7 +2,7 @@
 min_version = 4
 env_list =
     pre-commit,
-    mypy,
+    mypy-{all,api},
     py37,
     py38,
     py39,
@@ -33,12 +33,18 @@ deps = pre-commit
 commands = pre-commit run --all-files
 
 
-[testenv:mypy]
-description = Check types
+[testenv:mypy-all]
+description = Check package and API.
 deps = mypy
 commands = mypy src typing_examples.py
 
 
+[testenv:mypy-api]
+description = Check API types.
+deps = mypy
+commands = mypy typing_examples.py
+
+
 [testenv]
 description = Run tests and do NOT measure coverage.
 extras = tests