Added black formatter to seldon_core and testing folders

[axsaucedo]

I have ran the black formatter across the python and testing folders only.

[adriangonz]

Nice one @axsaucedo !

As part of this PR (or on subsequent ones) we could look at:

• Configuring pre-commit to run black locally on any commit which includes *.py files. The .pre-commit-config.yaml file got added already as part of Tidy up Java #951. black has support for pre-commit out of the box, so it would involve simply adding these lines.
• Adding a flake8 config to lint the *.py files. (we can get inspiration from black's). We can later run flake8 as part of the CI/CD pipeline.

[jklaise]

@adriangonz good ideas! In fact I'm keen to steal the pre-commit config for alibi once it's done for seldon-core... For flake8 config we have this in alibi and CI/CD on travis already.

[ukclivecox]

/approve

[seldondev]

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