New github actions structure (#658)

Author: sobolevn

.github/workflows/test.yml

@@ -1,6 +1,6 @@
 name: test
 
-on: [push, pull_request]
+on: [push, pull_request, workflow_dispatch]
 
 jobs:
   build:
@@ -21,22 +21,21 @@ jobs:
         curl -sSL \
           "https://raw.githubusercontent.com/python-poetry/poetry/master/get-poetry.py" | python
 
+        # Adding `poetry` to `$PATH`:
+        echo "$HOME/.poetry/bin" >> $GITHUB_PATH
+
     - name: Set up cache
       uses: actions/cache@v1
       with:
         path: .venv
         key: venv-${{ matrix.python-version }}-${{ hashFiles('poetry.lock') }}
     - name: Install dependencies
       run: |
-        source "$HOME/.poetry/env"
-
         poetry config virtualenvs.in-project true
         poetry install
 
     - name: Run tests
       run: |
-        source "$HOME/.poetry/env"
-
         poetry run flake8 .
         poetry run mypy returns ./tests/**/*.py
         # Different python versions are covered differently: