python: copy the current Python generator into a "pydantic v1" generator (#16656)

* Copy the current Python generator into a "pydantic v1" generator

This generator will be deprecated over time and the normal generator will
focus on Pydantic v2.

* add missing doc

samples/client/echo_api/python-pydantic-v1/.github/workflows/python.yml

@@ -0,0 +1,38 @@
+# NOTE: This file is auto generated by OpenAPI Generator.
+# URL: https://openapi-generator.tech
+#
+# ref: https://docs.github.com/en/actions/automating-builds-and-tests/building-and-testing-python
+
+name: openapi_client Python package
+
+on: [push, pull_request]
+
+jobs:
+  build:
+
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        python-version: ["3.7", "3.8", "3.9", "3.10", "3.11"]
+
+    steps:
+      - uses: actions/checkout@v3
+      - name: Set up Python ${{ matrix.python-version }}
+        uses: actions/setup-python@v4
+        with:
+          python-version: ${{ matrix.python-version }}
+      - name: Install dependencies
+        run: |
+          python -m pip install --upgrade pip
+          pip install flake8 pytest
+          if [ -f requirements.txt ]; then pip install -r requirements.txt; fi
+          if [ -f test-requirements.txt ]; then pip install -r test-requirements.txt; fi
+      - name: Lint with flake8
+        run: |
+          # stop the build if there are Python syntax errors or undefined names
+          flake8 . --count --select=E9,F63,F7,F82 --show-source --statistics
+          # exit-zero treats all errors as warnings. The GitHub editor is 127 chars wide
+          flake8 . --count --exit-zero --max-complexity=10 --max-line-length=127 --statistics
+      - name: Test with pytest
+        run: |
+          pytest