Make poetry use the correct python version

see actions/setup-python#374 (comment)

windows hosts cannot find python3.x in $PATH, but after
setup-python the python executable uses the correct version

.github/workflows/docs.yml

@@ -22,10 +22,11 @@ jobs:
         with:
           python-version: "3.7"
           cache: "poetry"
-          cache-dependency-path: "**/poetry.lock"
 
       - name: Install dependencies
-        run: poetry install
+        run: |
+          poetry env use "3.7"
+          poetry install
 
       - name: Deploy docs
         run: poetry run portray on_github_pages -f

.github/workflows/pypi_publish.yml

@@ -21,10 +21,11 @@ jobs:
         with:
           python-version: "3.7"
           cache: "poetry"
-          cache-dependency-path: "**/poetry.lock"
 
       - name: Install dependencies
-        run: poetry install
+        run: |
+          poetry env use "3.7"
+          poetry install
 
       - name: Bump version number
         run: poetry version ${{ github.event.release.tag_name }}

.github/workflows/run_tests.yml

@@ -26,10 +26,11 @@ jobs:
         with:
           python-version: ${{ matrix.python-version }}
           cache: "poetry"
-          cache-dependency-path: "**/poetry.lock"
 
       - name: Install dependencies
-        run: poetry install
+        run: |
+          poetry env use python
+          poetry install
 
       - name: Run Checks
         run: |

.github/workflows/test_pypi_publish.yml

@@ -21,10 +21,11 @@ jobs:
         with:
           python-version: "3.7"
           cache: "poetry"
-          cache-dependency-path: "**/poetry.lock"
 
       - name: Install dependencies
-        run: poetry install
+        run: |
+          poetry env use "3.7"
+          poetry install
 
       - name: Bump version number
         run: poetry version ${{ github.event.release.tag_name }}