Test Python 3.7 with a poetry that supports it (#21)

* Test Python 3.7 with a poetry that supports it

Poetry [dropped support for Python
3.7](python-poetry/poetry#7674) in [version
1.6.0](https://python-poetry.org/history/#160---2023-08-20). There's no
reason why this plugin cannot work in a python 3.7 environment, provided
a poetry that supports that environment is installed. Since poetry 1.5.1
was the last version to support Python 3.7 this change switches the test
setup to use poetry 1.5.1 in the case of the Python 3.7 branch of the
testing matrix.

* Clarify the specifics of Python 3.7 support

.github/workflows/build.yml

@@ -34,7 +34,12 @@ jobs:
         run: echo ::set-output name=version::$(python -c "import sys; print('-'.join(str(v) for v in sys.version_info))")
 
       - name: Install poetry
-        run: curl -sL curl -sL https://install.python-poetry.org | python - -y
+        run: |
+          if [[ "${{ matrix.python-version }}" == "3.7" ]]; then
+            curl -sSL https://install.python-poetry.org | python - --version 1.5.1 -y
+          else
+            curl -sSL https://install.python-poetry.org | python - -y
+          fi
 
       - name: Update PATH
         run: echo "$HOME/.local/bin" >> $GITHUB_PATH

README.md

@@ -4,7 +4,7 @@
 
 A [Poetry](https://python-poetry.org/) plugin that automatically loads environment variables from `.env` files into the environment before poetry commands are run.
 
-Supports Python 3.7+
+Supports Python 3.7+[^1]
 
 ```sh
 $ cat .env
@@ -37,6 +37,7 @@ PIPENV_DONT_LOAD_ENV | POETRY_DONT_LOAD_ENV
 
 ### Overriding existing environment variables
 
-By default, this plugin will override existing environment variables. This is because this plugin was built to make onboarding for users coming from `pipenv` as seamless as possible. If you want to prevent existing environment variables from being overridden, you can set the `POETRY_DOTENV_DONT_OVERRIDE` environment variable to `true`.[^1]
+By default, this plugin will override existing environment variables. This is because this plugin was built to make onboarding for users coming from `pipenv` as seamless as possible. If you want to prevent existing environment variables from being overridden, you can set the `POETRY_DOTENV_DONT_OVERRIDE` environment variable to `true`.[^2]
 
-[^1]: See [#16](https://github.com/mpeteuil/poetry-dotenv-plugin/pull/16) for background.
+[^1]: Python 3.7 is supported only when using Poetry < [1.6.0](https://python-poetry.org/history/#160---2023-08-20), which [dropped support for Python 3.7](https://github.com/python-poetry/poetry/pull/7674).
+[^2]: See [#16](https://github.com/mpeteuil/poetry-dotenv-plugin/pull/16) for background.