fix: default bridge and tests to project venv Python

Use .venv/bin/python3 by default, with PYTHON_PATH as an override, and update the setup script and docs to match the new virtualenv-based workflow.

src/keepass.ts

@@ -13,7 +13,7 @@ export class KeePassDatabase {
 	constructor(
 		private readonly path: string,
 		private readonly options: KeePassOpenOptions,
-		private readonly pythonPath = "python3",
+		private readonly pythonPath = process.env.PYTHON_PATH ?? ".venv/bin/python3",
 		private readonly bridgePath = new URL("./python/bridge.py", import.meta.url)
 	) {}