Fix app.blueprints type annotation

[hrimov]

Add quart.blueprints.Blueprint type annotation for the blueprints property instead of the inherited flask.sansio.blueprints.Blueprint type.

fixes #404

With a given MRE:

from quart import Quart
from quart.blueprints import Blueprint


app = Quart(__name__)
bp = Blueprint("test", __name__)
app.register_blueprint(bp)

# This works at runtime but fails type check
def process_blueprint(blueprint: Blueprint) -> None:
    print(f"Processing blueprint: {blueprint.name}")

# Type error here - blueprints.values() returns flask.sansio.blueprints.Blueprint
for blueprint in app.blueprints.values():
    process_blueprint(blueprint)  # Error: Expected quart.blueprints.Blueprint, got flask.sansio.blueprints.Blueprint

# Show inferred types
reveal_type(bp)  # Shows quart.blueprints.Blueprint
reveal_type(app.blueprints)  # Now it shows Dict[str,quart.blueprints.Blueprint] instead of Dict[str, flask.sansio.blueprints.Blueprint]