chore(example): migrate flask provider example

Following the changes to the FastAPI example, this migrates the Flask
provider example to the new structure. The example relies on the
consumer having published contracts, and the flask provider is verified
against those contracts.

Signed-off-by: JP-Ellis <josh@jpellis.me>

Author: JP-Ellis

examples/flask_provider/.flake8

@@ -0,0 +1,51 @@
+"""
+Flask provider example.
+
+This modules defines a simple
+[provider](https://docs.pact.io/getting_started/terminology#service-provider)
+with Pact. As Pact is a consumer-driven framework, the consumer defines the
+contract which the provider must then satisfy.
+
+The provider is the application which receives requests from another service
+(the consumer) and returns a response. In this example, we have a simple
+endpoint which returns a user's information from a (fake) database.
+
+Note that the code in this module is agnostic of Pact. The `pact-python`
+dependency only appears in the tests. This is because the consumer is not
+concerned with Pact, only the tests are.
+"""
+
+from __future__ import annotations
+
+from typing import Any
+
+from flask import Flask
+
+app = Flask(__name__)
+
+"""
+As this is a simple example, we'll use a simple dict to represent a database.
+This would be replaced with a real database in a real application.
+
+When testing the provider in a real application, the calls to the database
+would be mocked out to avoid the need for a real database. An example of this
+can be found in the test suite.
+"""
+FAKE_DB: dict[int, dict[str, Any]] = {}
+
+
+@app.route("/users/<uid>")
+def get_user_by_id(uid: int) -> dict[str, Any] | tuple[dict[str, Any], int]:
+    """
+    Fetch a user by their ID.
+
+    Args:
+        uid: The ID of the user to fetch
+
+    Returns:
+        The user data if found, HTTP 404 if not
+    """
+    user = FAKE_DB.get(uid)
+    if not user:
+        return {"error": "User not found"}, 404
+    return user