Update poetry test of oldest supported python version to 3.8 (depen…

…dabot#7691)

This test ensures that the installed version of `poetry` works as
expected with the oldest version of python currently supported by
Dependabot. It (hopefully) prevents us from installing newer poetry
versions that don't have support for older python versions.

In just a couple of weeks we will be deprecating Python 3.6/3.7... So
3.8 will become the oldest version of Python supported by Dependabot.

So update the test accordingly.

To update I ran the following commands:
```shell
bin/docker-dev-shell python
pyenv install 3.8.17  # matches the version available to be installed from the `python/Dockerfile`
PYENV_VERSION=3.8.17 pyenv exec pip install --upgrade pip
PYENV_VERSION=3.8.17 pyenv exec pip install poetry
PYENV_VERSION=3.8.17 pyenv exec poetry lock --no-update
```

python/spec/dependabot/python/file_updater/poetry_file_updater_spec.rb

@@ -119,10 +119,10 @@
       end
     end
 
-    context "with a supported python version", :slow do
-      let(:python_version) { "3.6.9" }
-      let(:pyproject_fixture_name) { "python_36.toml" }
-      let(:lockfile_fixture_name) { "python_36.lock" }
+    context "with the oldest python version currently supported by Dependabot", :slow do
+      let(:python_version) { "3.8.17" }
+      let(:pyproject_fixture_name) { "python_38.toml" }
+      let(:lockfile_fixture_name) { "python_38.lock" }
       let(:dependency) do
         Dependabot::Dependency.new(
           name: "django",

python/spec/fixtures/pyproject_files/python_36.toml → python/spec/fixtures/pyproject_files/python_38.toml

@@ -5,7 +5,7 @@ description = ""
 authors = ["Your Name <you@example.com>"]
 
 [tool.poetry.dependencies]
-python = "3.6.9"
+python = "3.8.17"
 requests = "^2.27.1"
 Django = "3.0"