[PW-6793] Add new CI and update tests

[jillingk]

Description
Created new CI pipeline with Tox to run the unittests on push and pull. Didn't add support for python 2.7 since that will be deprecated.

[wboereboom]

Could you please remove Python 2.7 from the README as well if it's deprecated

[michaelpaul]

Could you please remove Python 2.7 from the README as well if it's deprecated

@wboereboom I've suggested to deprecate Python 2.7. I think we should communicate that to merchants first and actually drop it on a next major release of the library.

[wboereboom]

@michaelpaul in that case let's just state now that it's end of support and it will no longer be maintained. Does that still warrant a major update?

[michaelpaul]

@michaelpaul in that case let's just state now that it's end of support and it will no longer be maintained. Does that still warrant a major update?

Yep, if a merchant application is still on Python 2, migrating to 3 could be a lot of work. Merchants don't expect to do such effort with minor/patch upgrades of the library.

@jillingk Can you please update the README.md about this:

• Python 2.7 (Deprecated) or 3.6