Commercetools

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- Fix cart create function using incorrect schema
- Add update actions to carts testing backend
- Add update actions to orders testing backend
- Regenerate code based on new commercetools-api-reference definition

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- Refactor the code generation module to split the types.py and schemas.py file
into multile submodules. This should be fully backwards compatible and from
the API perspective nothing should be changed, but it makes the generated
code more maintanable since we no longer have files with 40k lines.
- Use timezone aware timestamps in the mocking interface
- Rewrite the Paginator class. This is a backwards incompatible change but it
makes the API much cleaner. It now also supports slicing etc.
- Add a CursorPaginator class, which uses filtering instead of offsets for
pagination.

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- Money -> TypedMoney conversion not setting default to CENT_PRECISION
- Improve testing interface to only raise conflict error when the data is
actually changed

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- Tax related fields now parse rate as float instead of always being 0
- Support generating code for Python 3.7 and higher
- Add limited testing service for orders

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- Add `commercetools.predicate.QueryPredicate()` which can be used to construct
query predicate strings using Python code.
- Fix commercetools mock server

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- When creating the client the token url should now be a complete URL. When
using environment variables it will automatically append /oauth/token if the
url has no path specified. See 27
- Add a changelog :-)
- Add support for the product discounts api endpoints