Replace pycodestyle with flake8

[wxtim]

Other projects related to this one have been moving to using flake8.
This change proposed for the sake of consistency.

[MetRonnie]

Should we add flake8 to

isodatetime/setup.py

Line 98 in fbe537d

tests_require=['coverage', 'pytest>=5', 'pytest-cov', 'pytest-env'],

[oliver-sanders]

Yes but I let it slip since pyccodestyle wasn't there before.

If adding it there (good idea) add the tests_require to an all group so we can pip install -e .[all] as with other repos.

[wxtim]

I have now modified setup.py to include flake8 and the option to use pip install -e .[all]

[MetRonnie]

I learnt a thing or two about how pip install works from this 👍

[oliver-sanders]

Just waiting for tests to pass...

[wxtim]

I learnt a thing or two about how pip install works from this +1

Moi aussi!