Add tools to poetry

[jenshorn]

Enable isort, black and flake8 using poetry to make it easier to handle tools and their versions. Also aim to sync the config of isort and black during CI as they have different defaults. For example Black wants to have this formatting:

from tests.fixtures.sample_responses import (
    SAMPLE_CUSTOMER_DETAILS_JSON,
    SAMPLE_EMS_SETTINGS_RESPONSE,
    SAMPLE_FCRD_RESPONSE,
    SAMPLE_LOGIN_RESPONSE,
    SAMPLE_POWER_DATA_RESPONSE,
)

But isort wants this:

from tests.fixtures.sample_responses import (SAMPLE_CUSTOMER_DETAILS_JSON,
                                             SAMPLE_EMS_SETTINGS_RESPONSE,
                                             SAMPLE_FCRD_RESPONSE,
                                             SAMPLE_LOGIN_RESPONSE,
                                             SAMPLE_POWER_DATA_RESPONSE)