New Client Generator: Python uplink (+marshmallow)

Created by: cognifloyd

PR checklist

• Read the contribution guidelines.
• [partial] Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh, ./bin/security/{LANG}-petstore.sh and ./bin/openapi3/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
• Filed the PR against the correct branch: master, 3.4.x, 4.0.x. Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

Description of the PR

WIP python-uplink client SDK generator

This client SDK generator uses the uplink and marshmallow libraries. This began by forking the python client generator, discarding most of the python code, and then re-writing the python portion.

uplink handles the communication with the server using user-pluggable HTTP backends including requests, twisted, and aiohttp. The current python-client generator requires selecting the library to use at generation time, but uplink leaves such a choice up to the API SDK user instead of the API SDK author.

marshmallow handles the model de/serialization to/from namedtuple models, including validation at various points. Uplink comes with support for marshmallow so it was easiest to just use that instead of rolling a custom model library again.

The static auth method classes should, for the most part, be in the upstream uplink project. Most of them have been merged into master, but those changes have not made it into a release yet, so I included them here (I wrote them based on a few other classes in uplink which is under the MIT license).

Other marshmallow and uplink helper classes may also make their way upstream.

TODO:

• Add model generation support for composed schemas: All/Any/OneOf
• Add a Marshmallow Bytes field (String is unicode)
• Use Bytes field for types: file, binary, ByteArray, password
• Make sure model generation works for all cases including when an array or other primitive is the top level type in a schema.
• generate client lib tests (stubs?) that actually do something.
• provide defaultValue for: Date, DateTime, BigDecimal
• add option/switch(es) to allow serializing default values and/or deserializing missing values in client-side models (this generator leaves defaultValue inclusion to the API server)
• Add first class cookie support to uplink
• Add oauth support
• Fix the generated docs so they're actually helpful/informative. (So far, only the example was updated).
• Ensure that the partials loading works on windows (possibly by replaing / with \ when looking up partials on windows)