Add mypy cache to CI (#68)

Author: piotr-rarus

.github/workflows/build.yml

@@ -29,6 +29,12 @@ jobs:
         path: ~/.cache/pre-commit
         key: pre-commit-${{ runner.os }}-${{ steps.setup-python.outputs.python-version }}-${{ hashFiles('.github/hooks/.pre-commit-config.yml') }}
 
+    - name: Cache mypy
+      uses: actions/cache@v3
+      with:
+        path: .mypy_cache/
+        key: mypy_cache-${{ runner.os }}-${{ steps.setup-python.outputs.python-version }}-${{ hashFiles('poetry.lock') }}
+
     - name: Install dependencies
       if: steps.cached-poetry-dependencies.outputs.cache-hit != 'true'
       run: poetry install --no-interaction --no-root