Don't run mypy on pypy.

.github/workflows/pythonpackage.yml

@@ -21,13 +21,18 @@ jobs:
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip
-        pip install -U pip coverage codecov flake8 fastimport mypy
+        pip install -U pip coverage codecov flake8 fastimport
+    - name: Install mypy
+      run: |
+        pip install -U mypy
+      if: 'matrix.python-version != "pypy3"'
     - name: Style checks
       run: |
         python -m flake8
     - name: Typing checks
       run: |
         python -m mypy dulwich
+      if: 'matrix.python-version != "pypy3"'
     - name: Build
       run: |
         python setup.py build_ext -i