Re-implement API functions using requests

[rflprr]

Retire swagger auto-gen in favor of simpler implementation using requests.

Start by getting rid of:

• .swagger-codegen
• _swagger
• .swagger-codegen-ignore
• swagger-codegen-config.json
• swagger-swapi-def.json
• Makefile (update_swagger_codegen target)

The pattern I would like to encourage is one where:

1. Each module (*.py) file represents a section of the data.world api (e.g. projects, datasets, insights, etc). The ApiClient class, should aggregate all modules and serve as a point of access to all of them (e.g. ApiClient.projects, ApiClient.datasets, etc)
2. Each function follows a naming convention (TBD) and maps positional args to endpoint path parameters and keyword args to body parameters (see: https://github.com/datadotworld/target-datadotworld/blob/master/target_datadotworld/api_client.py#L164)
   2.1. If Content-Type needs to be defined (e.g. streams): TBD
   2.2. If upload parts need to defined (e.g. multi-part file upload): TBD
   2.3. If Accepts needs to be defined (e.g. sql): TBD
3. A request session object defines common headers such as Authorization and User-Agent (see: https://github.com/datadotworld/target-datadotworld/blob/master/target_datadotworld/api_client.py#L37)
4. HTTP 429 responses are automatically handled (see: https://github.com/datadotworld/target-datadotworld/blob/master/target_datadotworld/api_client.py#L366)
5. All validation and business rules are delegated to the server, and requests exceptions can be thrown as is (avoid anti-patterns like this: https://github.com/datadotworld/target-datadotworld/blob/master/target_datadotworld/api_client.py#L192)

Additionally:

1. Every function must include detailed docstrings (**kwargs documentation should point to the appropriate model doc, for example, https://apidocs.data.world/api/models/datasetcreaterequest)
2. Every function must be unit tested. Tests must assert that:
   2.1. HTTP request is composed with exactly the expected path, headers and body)
   2.2. Function returns the correct values upon success
   2.3. Function returns the correct errors upon failure

IMPORTANT: To maximize the benefits of these improvements, I would suggest breaking compatibility (i.e. release as v2.0).

[rflprr]

Py: Rewrite API wrappers