[Feature proposal: Integrate dependency support](https://github.com/actions/setup-python/issues/77#issue-593687811)

[Create-action]

I think a most users of this action run an "install dependencies" step immediately after. I think it would improve the developer experience if the "install dependencies" step was integrated into this action.

Current workflow:

    - name: Set up Python ${{ matrix.python-version }}
      uses: actions/setup-python@v1
      with:
        python-version: ${{ matrix.python-version }}
    - name: Install dependencies
      run: |
        python -m pip install --upgrade pip
        pip install -r requirements.txt

Option 1: Support installing packages with pip

    - name: Set up Python ${{ matrix.python-version }}
      uses: actions/setup-python@v1
      with:
        python-version: ${{ matrix.python-version }}
        pip-install: -r requirements.txt

        # or if you use a different package manager...
        # pip-install: pipenv
    # - name: Install dependencies
      # run: pipenv install

Option 2: Add full support for all package managers

    - name: Set up Python ${{ matrix.python-version }}
      uses: actions/setup-python@v1
      with:
        python-version: ${{ matrix.python-version }}
        # Pip
        package-manager: pip
        package-manager-args: -r requirements.txt
        # Pipenv
        package-manager: pipenv
        package-manager-args: --pre --dev --deploy
        # Poetry
        package-manager: poetry
        package-manager-args: --no-dev

What do you think? Option 1 is much smaller in scope than option 2, but I think both would be low maintenance (Python has a lot of package managers, but not that many).

[create-issue-branch]

Could not create branch issue-10-_Feature_proposal_Integrate_dependency_support_https_//github_com/actions/setup-python/issues/77#issue-593687811 due to: refs/heads/issue-10-Feature_proposal_Integrate_dependency_support_https//github_com/actions/issues/77#issue-593687811 is not a valid ref name.