Allow at() to be used with both contracts and blueprints

[benber86]

What I did

Allow for the use of at() with blueprint contracts. Currently:

@pytest.fixture
def token_blueprint() -> VyperContract:
    return TokenBlueprint.deploy_as_blueprint()

@pytest.fixture
def token_factory(token_blueprint) -> VyperContract:
    return TokenFactory.deploy(token_blueprint)


def test_deployment(token_factory, token_blueprint):
    token_address = token_factory.deploy_token("Snek", "SNK", 18, "Snek", "1")
    print(f"Token deployed at: {token_address}")
    factory = token_factory.at(token_factory.address) # This will work
    token = token_blueprint.at(token_address) # This will not

Right now token_blueprint.at(token_address) does not work and one would have to use token_blueprint.deployer.at(token_address) which is inconsistent with the behavior of regular VyperContracts.

How I did it

Moved at (which was only in VyperContract) to the _BaseVyperContract class

How to verify it

Description for the changelog

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